Add claude.md with project conventions and build instructions

claude.md

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+# EntityFramework.Exceptions
+
+Typed exception handling for Entity Framework Core. Converts database-specific errors into strongly-typed .NET exceptions instead of generic `DbUpdateException`.
+
+## Build & Test
+
+```bash
+dotnet restore                # Restore NuGet dependencies
+dotnet build --no-restore     # Build all projects
+dotnet test --no-restore      # Run tests (requires Docker for Testcontainers)
+```
+
+Tests use **Testcontainers** and require a running Docker daemon for containerized providers (SQL Server, PostgreSQL, MySQL, Oracle), each of which spins up its own container. **SQLite** tests run in-process and do not require Docker.
+
+## Project Structure
+
+The solution (`EntityFramework.Exceptions.slnx`) has two main layers:
+
+- **DbExceptionClassifier/** — Database-specific error code classification. Each provider implements `IDbExceptionClassifier` to map native error codes to a `DatabaseError` enum.
+  - `Common/` — `IDbExceptionClassifier` interface
+  - `SqlServer/`, `PostgreSQL/`, `MySQL/`, `MySQL.Pomelo/`, `Oracle/`, `Sqlite/` — Provider implementations
+
+- **EntityFramework.Exceptions/** — EF Core integration via interceptors. Catches `DbException`, classifies it, and throws a typed exception.
+  - `Common/` — Base `ExceptionProcessorInterceptor<T>`, exception classes (`UniqueConstraintException`, `CannotInsertNullException`, `MaxLengthExceededException`, `NumericOverflowException`, `ReferenceConstraintException`), `ExceptionFactory`
+  - `SqlServer/`, `PostgreSQL/`, `MySQL/`, `MySQL.Pomelo/`, `Oracle/`, `Sqlite/` — Provider-specific interceptors and `UseExceptionProcessor()` extension methods
+  - `Tests/` — xUnit test suite using Testcontainers
+
+- **Directory.Build.props** — Shared build properties (target framework, version, NuGet metadata). All non-Common projects automatically reference their corresponding Common project.
+
+## Architecture
+
+1. **Interceptor pattern**: `ExceptionProcessorInterceptor<TProviderException>` implements `IDbCommandInterceptor` and `ISaveChangesInterceptor`
+2. **Classification**: Each database provider has an `IDbExceptionClassifier` that maps native error codes to `DatabaseError` enum values
+3. **Factory**: `ExceptionFactory` creates the appropriate typed exception
+4. **Extension methods**: Each provider exposes `UseExceptionProcessor()` on `DbContextOptionsBuilder`
+
+## Code Conventions
+
+- **C# / .NET 8.0** with file-scoped namespaces, primary constructors, nullable reference types, and implicit usings
+- **Naming**: `[Database]ExceptionClassifier`, `[Database]ExceptionProcessorInterceptor`, `ExceptionProcessorExtensions.UseExceptionProcessor()`
+- **Test naming**: `[Scenario]Throws[ExceptionType]` (e.g., `UniqueColumnViolationThrowsUniqueConstraintException`)
+- **MySQL.Pomelo** shares source files with **MySQL** via `<Link>` in .csproj and uses `#if POMELO` preprocessor directive
+- Exception classes follow a standard pattern: inherit `DbUpdateException`, provide all standard constructor overloads, and optionally expose `ConstraintName`, `ConstraintProperties`, and `SchemaQualifiedTableName` properties
+
+## Testing Notes
+
+- Base test class `DatabaseTests` defines ~12 virtual test methods; provider-specific test classes inherit and override as needed
+- **SQLite** does not populate `ConstraintName`/`ConstraintProperties` and does not enforce numeric overflow
+- **SQL Server** skips numeric overflow tests (`ArgumentException` from SqlClient)
+- **MySQL** primary key violations do not populate constraint properties
+- Test fixtures use `IAsyncLifetime` for container lifecycle management