update: drop support for net48 in favor of netstandard2.1

- bumped LangVersion to 11.0
- added basic AGENTS.md

+semver: breaking

Author: mrdav30

AGENTS.md

@@ -0,0 +1,38 @@
+# FixedMathSharp agent instructions
+
+## Project intent and architecture
+- This repository is a deterministic fixed-point math library centered on `Fixed64` (`src/FixedMathSharp/Numerics/Fixed64.cs`) with Q32.32 representation (`SHIFT_AMOUNT_I = 32` in `src/FixedMathSharp/Core/FixedMath.cs`).
+- Most API surface lives in value-type numerics (`Fixed64`, `Vector2d`, `Vector3d`, `FixedQuaternion`, `Fixed3x3`, `Fixed4x4`) plus bounds types (`BoundingBox`, `BoundingSphere`, `BoundingArea`) implementing `IBound`.
+- Keep operations deterministic and allocation-light; many methods use `m_rawValue` and `[MethodImpl(MethodImplOptions.AggressiveInlining)]` for hot paths.
+- `FixedMath` and `FixedTrigonometry` are the shared algorithm backbones; extension classes in `src/FixedMathSharp/Numerics/Extensions/` are thin forwarding wrappers, not alternate implementations.
+
+## Build and test workflows
+- Solution: `FixedMathSharp.sln` with library project and test project.
+- Target frameworks are multi-targeted in both projects: `net48;net8`.
+- Typical local workflow:
+  - `dotnet restore`
+  - `dotnet build --configuration Debug --no-restore`
+  - `dotnet test --configuration Debug`
+- CI detail from `.github/workflows/dotnet.yml`:
+  - Linux runs `net48` tests via Mono + `xunit.console.exe` and `net8` via `dotnet test -f net8`.
+  - Windows runs `dotnet test` for both TFMs.
+- Packaging/versioning comes from `src/FixedMathSharp/FixedMathSharp.csproj`: GitVersion variables are consumed when present, otherwise version falls back to `0.0.0`.
+
+## Code conventions specific to this repo
+- Prefer `Fixed64` constants (`Fixed64.Zero`, `Fixed64.One`, `FixedMath.PI`) over primitive literals in math-heavy code.
+- Preserve saturating/guarded semantics in operators and math helpers (for example `Fixed64` add/sub overflow behavior).
+- When touching bounds logic, maintain cross-type dispatch shape in `Intersects(IBound)` and shared clamping projection via `IBoundExtensions.ProjectPointWithinBounds`.
+- Serialization compatibility is intentional:
+  - MessagePack attributes on serializable structs (`[MessagePackObject]`, `[Key]`) across TFMs.
+  - Conditional serializers in tests (`BinaryFormatter` for `NET48`, `System.Text.Json` for `NET8`).
+- `ThreadLocalRandom` is marked `[Obsolete]`; new deterministic RNG work should prefer `DeterministicRandom` and `DeterministicRandom.FromWorldFeature(...)` in `src/FixedMathSharp/Utility/DeterministicRandom.cs`.
+
+## Testing patterns to mirror
+- Tests are xUnit (`tests/FixedMathSharp.Tests`). Keep one feature area per test file (e.g., `Vector3d.Tests.cs`, `Bounds/BoundingBox.Tests.cs`).
+- Use helper assertions from `tests/FixedMathSharp.Tests/Support/FixedMathTestHelper.cs` for tolerance/range checks rather than ad-hoc epsilon logic.
+- For deterministic RNG changes, validate same-seed reproducibility and bounds/argument exceptions like in `DeterministicRandom.Tests.cs`.
+
+## Agent editing guidance
+- Keep public API shape stable unless the task explicitly requests API changes.
+- Match existing style (regions, XML docs, explicit namespaces, no implicit usings).
+- Make focused edits in the relevant numeric/bounds module and update corresponding tests in the parallel test file.

src/FixedMathSharp/FixedMathSharp.csproj

@@ -2,8 +2,8 @@
   <!-- Global Properties: General Configuration Settings -->
   <PropertyGroup>
     <!-- Define the target framework and compatibility -->
-    <LangVersion>9.0</LangVersion>
-    <TargetFrameworks>net48;net8</TargetFrameworks>
+    <LangVersion>11.0</LangVersion>
+    <TargetFrameworks>netstandard2.1;net8</TargetFrameworks>
     <!-- Versioning and Build Configuration -->
     <!-- Set SemVer to GitVersion_FullSemVer if available, otherwise fallback to 0.0.0 -->
     <SemVer Condition="'$(GitVersion_FullSemVer)' != ''">$(GitVersion_FullSemVer)</SemVer>

tests/FixedMathSharp.Tests/FixedMathSharp.Tests.csproj

@@ -1,12 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <LangVersion>9.0</LangVersion>
-    <TargetFrameworks>net48;net8</TargetFrameworks>
+    <LangVersion>11.0</LangVersion>
+    <TargetFrameworks>net8</TargetFrameworks>
     <ImplicitUsings>disable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
 	<Configurations>Debug;Release</Configurations>
+	<OutputType>Exe</OutputType>
   </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\..\src\FixedMathSharp\FixedMathSharp.csproj">