docs: record final bucket CORS hardening design

[Vonng]

Summary

• add the English and Chinese public design record for per-bucket CORS site-replication convergence
• record the zero-baseline C-prime register, durable tombstones, exact source timestamps, deterministic equal-time conflicts, distributed CORS lock, full-state status/heal, initial-sync and restart semantics
• add the English and Chinese strict wire-contract record covering XML shape, Unicode IDs, exact methods, checksums, MaxAge presence, single-star matching, wildcard credentials, and Origin: null
• document legacy-invalid metadata fail-closed recovery, mixed-version/downgrade boundaries, inherited #76/#77 scope, and the remaining release gates
• keep implementation, merge, deployment, tag, image, and production claims separate

Evidence

• server implementation: pgsty/silo PR #80, final head 91d909175
• final B2+B3 code commit: 0eebc928f
• server PR checks: 8/8 green after integration
• local docs make check: warning-fatal Hugo build and 473,129 rendered internal references pass
• rendered pages: EN 1175, ZH 1177
• local browser QA: replication and wire-contract EN/ZH routes render with correct language/title/status and no broken images
• real local two-site acceptance and raw SigV4 protocol probes are recorded as local evidence only, not deployment or production proof

Refs pgsty/silo#75.

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