Add AI Instruction Files for GitHub Copilot

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+# DynamicData — AI Instructions
+
+## What is DynamicData?
+
+DynamicData is a reactive collections library for .NET, built on top of [Reactive Extensions (Rx)](https://github.com/dotnet/reactive). It provides `SourceCache<TObject, TKey>` and `SourceList<TObject>` — observable data collections that emit **changesets** when modified. These changesets flow through operator pipelines (Sort, Filter, Transform, Group, Join, etc.) that maintain live, incrementally-updated views of the data.
+
+DynamicData is used in production by thousands of applications. It is the reactive data layer for [ReactiveUI](https://reactiveui.net/), making it foundational infrastructure for the .NET reactive ecosystem.
+
+## Cache vs List — Two Collection Types
+
+DynamicData provides two parallel collection types. **Choose the right one — they are not interchangeable.**
+
+| | **Cache** (`SourceCache<T, TKey>`) | **List** (`SourceList<T>`) |
+|---|---|---|
+| **Identity** | Items identified by unique key | Items identified by index position |
+| **Duplicates** | Not allowed (key must be unique) | Allowed (same item at multiple positions) |
+| **Ordering** | Unordered by default (Sort adds ordering) | Inherently ordered (like `List<T>`) |
+| **Best for** | Entities with IDs, lookup by key | Ordered sequences, duplicates OK |
+| **Change types** | Add, Update, Remove, Refresh, Moved | Add, AddRange, Replace, Remove, RemoveRange, Moved, Refresh, Clear |
+| **Changeset** | `IChangeSet<TObject, TKey>` | `IChangeSet<T>` |
+
+**Rule of thumb:** If your items have a natural unique key (ID, name, etc.), use **Cache**. If order matters and/or duplicates are possible, use **List**. Cache is used far more often in practice.
+
+See `.github/instructions/dynamicdata-cache.instructions.md` for the complete cache operator reference.
+
+See `.github/instructions/dynamicdata-list.instructions.md` for the complete list operator reference.
+
+## Why Performance Matters
+
+Every item flowing through a DynamicData pipeline passes through multiple operators. Each operator processes changesets — not individual items — so a single cache edit with 1000 items creates a changeset that flows through every operator in the chain. At library scale:
+
+- **Per-item overhead compounds**: 1 allocation × 10 operators × 1000 items × 100 pipelines = 1M allocations per batch
+- **Lock contention is the bottleneck**: operators serialize access to shared state. Minimizing lock hold time is a core design goal.
+- **Prefer value types and stack allocation**: use structs, `ref struct`, `Span<T>`, and avoid closures in hot paths where possible
+
+When optimizing, measure allocation rates and lock contention, not just wall-clock time.
+
+## Why Rx Contract Compliance is Critical
+
+DynamicData operators compose — the output of one is the input of the next. If any operator violates the Rx contract (e.g., concurrent `OnNext` calls, calls after `OnCompleted`), every downstream operator can corrupt its internal state. This is not a crash — it's silent data corruption that manifests as wrong results, missing items, or phantom entries. In a reactive UI, this means the user sees stale or incorrect data with no error message.
+
+See `.github/instructions/rx.instructions.md` for comprehensive Rx contract rules, scheduler usage, disposable patterns, and a complete standard Rx operator reference.
+
+## Breaking Changes
+
+DynamicData follows [Semantic Versioning (SemVer)](https://semver.org/). Breaking changes **are possible** in major version bumps, but they are never done lightly. This library has thousands of downstream consumers — every breaking change has a blast radius.
+
+**Rules:**
+- Breaking changes require a major version bump. **You MUST explicitly call out any potentially breaking change to the user** before making it — even if you think it's minor. Let the maintainers decide.
+- Prefer non-breaking alternatives first: new overloads, new methods, optional parameters with safe defaults.
+- When a breaking change is justified, mark the old API with `[Obsolete("Use XYZ instead. This will be removed in vN+1.")]` in the current version and remove it in the next major.
+- Behavioral changes (different ordering, different filtering semantics, different error propagation) are breaking even if the signature is unchanged. Call these out.
+- Internal types (`internal` visibility) can change freely — they are not part of the public contract.
+
+**What counts as breaking:**
+- Changing the signature of a public extension method (parameters, return type, generic constraints)
+- Changing observable behavior (emission order, filtering semantics, error/completion propagation)
+- Removing or renaming public types, methods, or properties
+- Adding required parameters to existing methods
+- Changing the default behavior of an existing overload
+
+## Maintaining These Instructions
+
+These instruction files are living documentation. **They must be kept in sync with the code.**
+
+- When a **new operator** is added, it **MUST** be added to the appropriate instruction file (`dynamicdata-cache.instructions.md` or `dynamicdata-list.instructions.md`) with its change reason handling table.
+- When an **operator's behavior changes**, update its table in the instruction file.
+- When a **new test utility** is added, update the test utilities reference in the main instructions and the appropriate `testing-*.instructions.md`.
+- When a **new domain type** is added to `Tests/Domain/`, add it to the Domain Types section.
+
+## Repository Structure
+
+```
+src/
+├── DynamicData/                    # The library
+│   ├── Cache/                      # Cache (keyed collection) operators
+│   │   ├── Internal/               # Operator implementations (private)
+│   │   ├── ObservableCache.cs      # Core observable cache implementation
+│   │   └── ObservableCacheEx.cs    # Public API: extension methods for cache operators
+│   ├── List/                       # List (ordered collection) operators
+│   │   ├── Internal/               # Operator implementations (private)
+│   │   └── ObservableListEx.cs     # Public API: extension methods for list operators
+│   ├── Binding/                    # UI binding operators (SortAndBind, etc.)
+│   ├── Internal/                   # Shared internal infrastructure
+│   └── Kernel/                     # Low-level types (Optional<T>, Error<T>, etc.)
+├── DynamicData.Tests/              # Tests (xUnit + FluentAssertions)
+│   ├── Cache/                      # Cache operator tests
+│   ├── List/                       # List operator tests
+│   └── Domain/                     # Test domain types using Bogus fakers
+```
+
+## Operator Architecture Pattern
+
+Most operators follow the same two-part pattern:
+
+```csharp
+// 1. Public API: extension method in ObservableCacheEx.cs (thin wrapper)
+public static IObservable<IChangeSet<TDest, TKey>> Transform<TSource, TKey, TDest>(
+    this IObservable<IChangeSet<TSource, TKey>> source,
+    Func<TSource, TDest> transformFactory)
+{
+    return new Transform<TDest, TSource, TKey>(source, transformFactory).Run();
+}
+
+// 2. Internal: sealed class in Cache/Internal/ with a Run() method
+internal sealed class Transform<TDest, TSource, TKey>
+{
+    public IObservable<IChangeSet<TDest, TKey>> Run() =>
+        Observable.Create<IChangeSet<TDest, TKey>>(observer =>
+        {
+            // Subscribe to source, process changesets, emit results
+            // Use ChangeAwareCache<T,K> for incremental state
+            // Call CaptureChanges() to produce the output changeset
+        });
+}
+```
+
+**Key points:**
+- The extension method is the **public API surface** — keep it thin
+- The internal class holds constructor parameters and implements `Run()`
+- `Run()` returns `Observable.Create<T>` which **defers subscription** (cold observable)
+- Inside `Create`, operators subscribe to sources and wire up changeset processing
+- `ChangeAwareCache<T,K>` tracks incremental changes and produces immutable snapshots via `CaptureChanges()`
+- Operators must handle all change reasons: `Add`, `Update`, `Remove`, `Refresh`
+
+## Thread Safety in Operators
+
+When an operator has multiple input sources that share mutable state:
+- All sources must be serialized through a shared lock
+- Use `Synchronize(gate)` with a shared lock object to serialize multiple sources
+- Keep lock hold times as short as practical
+
+When operators use `Synchronize(lock)` from Rx:
+- The lock is held during the **entire** downstream delivery chain
+- This ensures serialized delivery across multiple sources sharing a lock
+- Always use a private lock object — never expose it to external consumers
+
+## Testing
+
+**All new code MUST come with unit tests that prove 100% correctness. All bug fixes MUST include a regression test that reproduces the bug before verifying the fix.** No exceptions. Untested code is broken code — you just don't know it yet.
+
+### Frameworks and Tools
+
+- **xUnit** — test framework (`[Fact]`, `[Theory]`, `[InlineData]`)
+- **FluentAssertions** — via the `AwesomeAssertions` NuGet package (`.Should().Be()`, `.Should().BeEquivalentTo()`, etc.)
+- **Bogus** — fake data generation via `Faker<T>` in `DynamicData.Tests/Domain/Fakers.cs`
+- **TestSourceCache<T,K>** and **TestSourceList<T>** — enhanced source collections in `Tests/Utilities/` that support `.Complete()` and `.SetError()` for testing terminal Rx events
+
+### Test File Naming and Organization
+
+Tests live in `src/DynamicData.Tests/` mirroring the library structure:
+- `Cache/` — cache operator tests (one fixture class per operator, e.g., `TransformFixture.cs`)
+- `List/` — list operator tests
+- `Domain/` — shared domain types (`Person`, `Animal`, `AnimalOwner`, `Market`, etc.) and Bogus fakers
+- `Utilities/` — test infrastructure (aggregators, validators, recording observers, stress helpers)
+
+Naming convention: `{OperatorName}Fixture.cs`. For operators with multiple overloads, use partial classes: `FilterFixture.Static.cs`, `FilterFixture.DynamicPredicate.IntegrationTests.cs`, etc.
+
+For cache-specific testing patterns (observation patterns, changeset assertions, stub pattern), see `.github/instructions/testing-cache.instructions.md`.
+
+For list-specific testing patterns, see `.github/instructions/testing-list.instructions.md`.
+
+### Testing the Rx Contract
+
+`.ValidateSynchronization()` detects Rx contract violations. It tracks in-flight notifications with `Interlocked.Exchange` — if two threads enter `OnNext` simultaneously, it throws `UnsynchronizedNotificationException`. It uses raw observer/observable types to bypass Rx's built-in safety guards so violations are surfaced, not masked.
+
+```csharp
+source.Connect()
+    .Transform(x => new ViewModel(x))
+    .ValidateSynchronization()    // THROWS if concurrent delivery detected
+    .RecordCacheItems(out var results);
+```
+
+### Writing Regression Tests for Bug Fixes
+
+Every bug fix **must** include a test that:
+1. **Reproduces the bug** — the test fails without the fix
+2. **Verifies the fix** — the test passes with the fix
+3. **Is named descriptively** — describes the scenario, not the bug ID
+
+```csharp
+// GOOD: describes the scenario that was broken
+[Fact]
+public void RemoveThenReAddWithSameKey_ShouldNotDuplicate()
+
+// BAD: meaningless to future readers
+[Fact]
+public void FixBug1234()
+```
+
+### Stress Test Principles
+
+- Use `Barrier` for simultaneous start — maximizes contention. Include the main thread in participant count.
+- Use deterministic data so failures are reproducible. Use `Bogus.Randomizer` with a fixed seed — **never `System.Random`**.
+- Assert the **exact final state**, not just "count > 0".
+- Use `Task.WhenAny(completed, Task.Delay(timeout))` to detect deadlocks with a meaningful timeout.
+- Include mixed operations: adds, updates, removes, property mutations, dynamic parameter changes.
+- Use the `StressAddRemove` extension methods in `Tests/Utilities/` for standard add/remove patterns with timed removal.
+
+### Test Utilities Reference
+
+The `Tests/Utilities/` directory provides powerful helpers — **use them** instead of reinventing:
+
+| Utility | Purpose |
+|---------|---------|
+| `ValidateSynchronization()` | Detects concurrent `OnNext` — Rx contract violation |
+| `ValidateChangeSets(keySelector)` | Validates structural integrity of every changeset |
+| `RecordCacheItems(out results)` | Cache recording observer with keyed + sorted tracking |
+| `RecordListItems(out results)` | List recording observer |
+| `TestSourceCache<T,K>` | SourceCache with `.Complete()` and `.SetError()` support |
+| `TestSourceList<T>` | SourceList with `.Complete()` and `.SetError()` support |
+| `StressAddRemove` extensions | Add/remove stress patterns with timed automatic removal |
+| `ForceFail(count, exception)` | Forces an observable to error after N emissions |
+| `Parallelize(count, parallel)` | Creates parallel subscriptions for stress testing |
+| `ObservableSpy` | Diagnostic logging for pipeline debugging |
+| `FakeScheduler` | Controlled scheduler for time-dependent tests |
+| `Fakers.*` | Bogus fakers for `Person`, `Animal`, `AnimalOwner`, `Market` |
+
+### Domain Types
+
+Shared domain types in `Tests/Domain/`:
+
+- **`Person`** — `Name` (key), `Age`, `Gender`, `FavoriteColor`, `PetType`. Implements `INotifyPropertyChanged`.
+- **`Animal`** — `Name` (key), `Type`, `Family` (enum: Mammal, Reptile, Fish, Amphibian, Bird)
+- **`AnimalOwner`** — `Name` (key), `Animals` (ObservableCollection). Ideal for `TransformMany`/`MergeManyChangeSets` tests.
+- **`Market`** / **`MarketPrice`** — financial-style streaming data tests
+- **`PersonWithGender`**, **`PersonWithChildren`**, etc. — transform output types
+
+Generate test data with Bogus:
+```csharp
+var people = Fakers.Person.Generate(100);
+var animals = Fakers.Animal.Generate(50);
+var owners = Fakers.AnimalOwnerWithAnimals.Generate(10);  // pre-populated with animals
+```
+
+### Test Anti-Patterns
+
+**❌ Testing implementation details instead of behavior:**
+```csharp
+// BAD: message count is an implementation detail — fragile
+results.Messages.Count.Should().Be(3);
+// GOOD: test the observable behavior and final state
+results.Data.Count.Should().Be(expectedCount);
+results.Data.Items.Should().BeEquivalentTo(expectedItems);
+```
+
+**❌ Using `Thread.Sleep` for timing:**
+```csharp
+// BAD: flaky and slow
+Thread.Sleep(1000);
+// GOOD: use test schedulers or deterministic waiting
+var scheduler = new TestScheduler();
+scheduler.AdvanceBy(TimeSpan.FromSeconds(1).Ticks);
+```
+
+**❌ Ignoring disposal:**
+```csharp
+// BAD: leaks subscriptions, masks errors
+var results = source.Connect().Filter(p => true).AsAggregator();
+// GOOD: using ensures cleanup even if assertion throws
+using var results = source.Connect().Filter(p => true).AsAggregator();
+```
+
+**❌ Non-deterministic data without seeds:**
+```csharp
+// BAD: failures aren't reproducible across runs
+var random = new Random();
+// GOOD: use Bogus Randomizer with a fixed seed
+var randomizer = new Randomizer(42);
+var people = Fakers.Person.UseSeed(randomizer.Int()).Generate(100);
+```