feat(xlang): refine xlang api and enum serialization

[chaokunyang]

Why?

• Java enums needed a stable numeric tag option for cross-language and schema-evolution scenarios that should not depend on declaration order.
• The xlang implementation guide and related docs/specs had drifted from the current runtime architecture and API surface.

What does this PR do?

• Remove ForyBuilder.Xlang(bool) from the C# public API, make Config builder-created only, and make C# always emit and require the xlang frame header.
• Add Java @ForyEnumId support so enums can use a single annotated field, a zero-argument getter, or per-constant annotations for stable numeric tags, with dense-array lookup for small IDs and sparse-map lookup for larger IDs.
• Add Java serializer and xlang tests covering default ordinal behavior, explicit enum IDs, validation failures, and cross-language enum evolution with stable IDs.
• Refresh the C# guides and Java/xlang specs, including a rewrite of the xlang implementation guide around the current Fory / WriteContext / ReadContext / RefWriter / RefReader architecture.

Related issues

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 of the final clean AI review results from both fresh reviewers on the current PR diff or current HEAD after the latest code changes.

Does this PR introduce any user-facing change?

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

Benchmark

N/A