feat(xlang): add bfloat16 support

[chaokunyang]

Why?

Add cross-language bfloat16 support alongside the existing reduced-precision float16 support so reduced-precision scalars and dense arrays can round-trip across runtimes with spec-backed behavior and documentation.

What does this PR do?

• adds bfloat16 scalar and array/list carriers plus serializer/type-resolution wiring in Java, C++, C#, Python, Go, and Rust, with the generated/object-codec paths updated where needed
• extends xlang and schema-evolution handling for reduced-precision fields, including array skip logic and field-ordering/type-id fixes required for consistent cross-language behavior
• adds cross-language coverage for reduced-precision structs and updates runtime tests across the touched implementations
• refreshes the xlang specification, type-mapping docs, and language guides/READMEs to document bfloat16 and reduced-precision array support

Related issues

#1017
Closes #3286
Closes #3283
Closes #3285

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?

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