Refactor source generation and improve generated mapping support

[ycherkes]

Refactor source generation and improve generated mapping support

Summary

This PR refactors AlephMapper's Roslyn generator around mapper-scoped incremental outputs, strengthens generated-code diagnostics and nullable handling, and expands support for complex expression and generic mapping scenarios. It also updates generated attributes, documentation, samples, and test coverage.

Breaking change

[Expressive] has been renamed to [Projectable] with no compatibility attribute. Replace every [Expressive] use with [Projectable].

Generated mappings now include the assembly revision in their GeneratedCode version metadata. Local builds emit 0.7.0.0; CI-packaged analyzers emit 0.7.0.<run-number>. Consumers that inspect this attribute using an exact version string must account for the revision component.

Highlights

Incremental generator architecture

• Replaces compiler-object-heavy cached models with value-oriented, mapper-scoped generation results.
• Narrows syntax candidate discovery to relevant mapper methods and attributes.
• Centralizes mapper generation, mapping lookup, diagnostics, and generated-file rendering.
• Separates source-only incremental output from compilation-dependent diagnostic rebinding, so unrelated compilation changes do not feed into AddSource.
• Projects generated source data into a dedicated value with source-only equality; diagnostic-only changes (including source-location shifts) no longer invalidate AddSource.
• Removes obsolete lookup and diagnostic paths while preserving packaging behavior.

Generated API and diagnostics

• Embeds generated configuration types as internal implementation details rather than exposing them through consumer APIs.
• Marks generated sources explicitly and improves attribute defaults and nullable contracts to eliminate generator and generated-attribute warnings.
• Adds analyzer release tracking and improves diagnostic descriptions and metadata.
• Introduces AM0018: [Adapt] on a generic mapping method is skipped because the generic mapper already provides the polymorphic behavior.

Mapping and expression generation

• Generates generic Projectable expression companions and Updatable overloads with the original type parameters and where constraints.
• Distinguishes generic method arity during generated-member conflict detection.
• Preserves nullable type information when rewriting null-conditional expressions, avoiding false nullable warnings in generated expressions.
• Preserves source nullable context around each generated member, including nullable-disabled sources, and validates the generated nullable-disabled fixture for CS86xx warnings.
• Avoids conflicting file-level nullable directives by emitting the appropriate #nullable state only around each generated member.
• Emits null-forgiving suppression only when nullable warnings are enabled; nullable-disabled output retains typed null casts without redundant ! operators.
• Centralizes nullable behavior in one effective policy per source mapping method, resolving annotation and warning inheritance independently and reusing it for type rendering, lowering, adaptation, updates, and member directives.
• Round-trips adaptation diagnostic source locations with their original file and line spans.
• Preserves diagnostic descriptor metadata and formatted messages when diagnostics cross the incremental-generation boundary.
• Rebinds reconstructed diagnostics to their original syntax trees at emission time, preserving source-based controls such as #pragma warning disable.
• Corrects the AM0004 crash-report help link.
• Allows ordinary external helper methods with expression bodies to be inlined without applying mapper candidate-discovery restrictions.
• Improves null-conditional, conditional-expression, switch-expression, extension-method, and invocation rewriting.
• Improves conditional and switch formatting while preserving single-line versus multi-line source intent.
• Emits collision-safe global::-qualified type references in generated declarations, expressions, object creation, casts, and constraints while retaining C# keyword types and generic parameters, and omits the redundant System.CodeDom.Compiler using.

Tests, samples, and documentation

• Expands incremental-generator, diagnostic, generic mapping, null-handling, conditional-formatting, adaptation, and inlining coverage.
• Clarifies incremental tracking coverage: unchanged values are tested for stable generation results, without claiming that the test proves a cache hit.
• Adds coverage proving that a diagnostic-location-only change leaves generated source text and the source-output projection unchanged.
• Refreshes generated baselines and integration fixtures.
• Updates Adapt documentation, README content, and sample mappers.
• Bumps the package version to 0.7.0.

CI and packaging

• Adds a GitHub Actions workflow that restores, builds, tests, and packs the solution on pull requests, pushes to main, and manual dispatch.
• Pins the .NET SDK, caches NuGet packages, and uploads TRX test reports alongside the generated package artifact.
• Restores the generated package into a minimal consumer project and compiles a [Projectable] mapper through the packaged analyzer.
• Stops automatically packing during ordinary builds; packaging is now an explicit release/CI step.
• Uses the GitHub Actions run number as the assembly-version revision; NuGet package versioning remains independently stable.

Generator reliability

• Compiles generated syntax trees as part of the source-generator baseline tests, while allowing fixture-only dependency errors to remain isolated.
• Re-throws unexpected generator exceptions in DEBUG builds instead of suppressing them after diagnostic creation.
• Derives generated-code version metadata from assembly metadata, including the build revision, avoiding a second hard-coded package version.
• Cleans up nullable contracts in samples and test fixtures, and preserves nullability in generated signatures and expressions without global warning suppressions.

Validation

• dotnet build AlephMapper.slnx --configuration Release --no-restore — succeeded with 0 warnings and 0 errors
• dotnet run --project tests/AlephMapper.Tests/AlephMapper.Tests.csproj -c Release --no-restore — 58 passed
• dotnet test AlephMapper.slnx --configuration Release --no-build --no-restore -- --report-trx --results-directory artifacts/test-results — 95 passed
• dotnet pack source/AlephMapper.csproj --configuration Release --no-build --no-restore --output artifacts/nuget — produced AlephMapper.0.7.0.nupkg
• dotnet test AlephMapper.slnx --no-restore — 96 passed