feat(csharp): support generated class inheritance

[chaokunyang]

Why?

Generated C# serializers previously modeled only members declared by the concrete class. Base-class
wire members could be omitted from field ordering, schema metadata, reads, and writes, while private
parent storage could not be included correctly when the parent came from another compilation.
Third-party base classes also need an explicit, application-owned way to describe inaccessible
fields without inspecting private assembly metadata.

What does this PR do?

• Adds declaration-owned hierarchy provider contracts for ordinary inheritable classes. A concrete
  serializer flattens the immediate parent provider's wire members with its own members and composes
  cumulative shallow-storage bytes without calling a parent serializer.
• Separates the flattened wire-member model from shallow instance-storage accounting so inherited
  schema behavior and graph-memory reservation each have one clear owner.
• Adds exact external class field mappings through TargetDeclaringType and TargetMemberName, plus
  BaseOnly declarations for unmodifiable third-party hierarchy prefixes and storage-only fields.
• Validates referenced provider identity, descriptor counts and ordinals, accessor signatures,
  accessibility, generic construction, and override/member identities before generating child code.
• Splits ForyModelGenerator into focused partial files for analysis, hierarchy composition,
  external mappings, models, and emission while retaining one generation path.
• Adds same-compilation, cross-assembly, external private-field, graph-memory, generator diagnostic,
  and C# xlang coverage, and updates the C# guides and xlang implementation specification.

Related issues

N/A

AI Contribution Checklist

• Substantial AI assistance was used in this PR: yes / no
• If yes, I included a completed AI Contribution Checklist in this PR description and the required AI Usage Disclosure.
• If yes, my PR description includes the required ai_review summary and screenshot evidence or equivalent persisted links of the final clean AI review results from both fresh reviewers described in AI_POLICY.md, the Fory-guided reviewer and the independent general reviewer, on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

Yes. Annotated C# class hierarchies now serialize inherited fields. External serializer declarations
gain BaseOnly, TargetDeclaringType, and TargetMemberName for explicit third-party class
hierarchy and private-field mappings. The shared binary protocol encoding is unchanged.

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

The existing C# external-equivalence benchmark model now declares exact backing storage through
storage-only external mappings. This PR does not add a benchmark harness or a separate inheritance
benchmark path.