[sdk-client S7] feat(sdk): ship locale as the non-auth contribution proof

[rickylabs]

Part of #1348. Implements RFC 0001 stage 6.

Summary

Prove that the contribution seam is general rather than auth-specific by shipping a locale contribution that owns accept-language, participates in typed context composition, and demonstrates cache-safe partitioning and deterministic conflict behavior.

Acceptance

• Locale contribution owns accept-language through the RFC 0001 descriptor contract.
• Duplicate ownership and reserved-header conflicts fail deterministically.
• Direct-call and query-factory behavior follows the declared cache partition law.
• Generated clients preserve the inferred locale context type.
• Tests cover composition order, retries, cancellation, cache keys, and redaction.
• Documentation includes auth and non-auth examples.
• Required SDK/plugin gates and IMPL-EVAL pass.