Isolate SaveChanges audit state per DbContext

[cdcavell]

Summary

• isolate mutable SaveChanges audit state by ApplicationDbContext instance
• route save lifecycle callbacks through a scoped coordinator that maintains one existing ApplicationSaveChangesPipeline per context using weak context keys
• add IApplicationMutationAuditReceiptRegistry for explicit context-specific receipt lookup with factory-created contexts
• keep the normal IApplicationMutationAuditReceiptAccessor bound to the scoped ApplicationDbContext
• preserve the existing interceptor, audit-store contracts, and single-context pipeline behavior

Tests

Added regression coverage that:

• creates two ApplicationDbContext instances from IDbContextFactory<ApplicationDbContext> in one scope
• pauses the first context during asynchronous audit append while the second context completes its save
• verifies distinct audit rows and mutation batch identifiers
• verifies each context resolves only its own completed receipt
• verifies audited values do not cross between contexts
• verifies the updated DI registrations and disabled-provider behavior
• disables SQLite pooling in the temporary-file test fixture for reliable Windows cleanup

Documentation

Added guidance covering:

• context-keyed audit state isolation
• receipt lookup for scoped and factory-created contexts
• weak-key lifetime behavior
• EF Core's prohibition on concurrent operations against the same context instance
• concurrency responsibilities for custom mutable scoped audit services

Validation

GitHub Actions CI passed:

• solution build and analyzer validation
• dotnet format verification
• full test suite
• coverage generation and repository threshold enforcement
• security-critical coverage thresholds
• dependency review
• CodeQL analysis
• template smoke tests on Ubuntu, Windows, and macOS
• Docker build, Compose startup, and health verification
• non-default no-auth/SQL Server scaffold validation

Fixes / Closes #374