feat(ai): support partial lexical block translations

[Innei]

Summary

• Add partial lexical translation construction and lexical block reuse helpers.
• Restrict partial translation reads to stale translations and fingerprint lexical property segments.
• Add focused behavior coverage and design documentation for the partial Lexical translation path.

Validation

• Not rerun during PR creation; prior review output reported focused tests plus lint/type checks passing.

[safedep]

SafeDep Report Summary

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