feat(python): support nested field schema encodings

[chaokunyang]

Why?

Python currently loses declared integer encoding aliases inside nested containers, so schemas such as Dict[pyfory.fixed_int32, List[pyfory.tagged_int64]] can fall back to runtime inference or incompatible local assignment behavior. This breaks cross-language schema fidelity for nested map/list/set elements, especially in compatible mode.

What does this PR do?

• Preserves declared Python field schema metadata for nested container keys, values, and elements in both pure Python and Cython serializers.
• Extends primitive container fast paths to write/read fixed, varint, tagged, signed, unsigned, bool, and float schema-owned primitive encodings directly.
• Adds compatible-read assignment planning and validation so remote nested integer encodings are consumed with remote metadata and assigned only when they satisfy the local schema.
• Updates Python code generation to keep nested integer schema aliases in generic containers instead of converting those lists to numpy array hints.
• Adds Python unit coverage, compiler generation coverage, and Java/Python xlang tests for signed and unsigned nested annotated containers.
• Documents nested Python collection aliases and compatible-read behavior in README and Python guide pages.

Related issues

#1017
#3630
#3625
#3630
#3636
#3639
#3640
#3642

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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