docs: expand CopyObject checksum design

[Vonng]

Summary

• expand the existing CopyObject checksum page from a short implementation note into a complete bilingual design record
• reconstruct the discovery path from #46 to #63 and distinguish the demonstrated transformed-stream bug from the unconfirmed missed-prefix hypothesis
• define the logical-object, ETag, storage-reader, erasure-bitrot, encryption, and compression-index integrity domains
• compare and reject object-layer fallback, transformed-stream hashing, ciphertext hashing, rereads, whole-object buffering, unconditional source-checksum reuse, and a second abstraction
• document the selected checksumReader design step by step and prove byte-domain correctness, EOF completeness, ETag preservation, storage-layer independence, encryption independence, and backward compatibility
• record the #67 and #68 adversarial follow-ups, the complete test matrix, mixed-version behavior, historical-object verification/remediation, release boundaries, and cross-repository ownership

Bilingual parity

• English and Chinese files are both 362 lines
• all 13 second-level sections use the same anchors and order
• both versions contain the same external link set and decision/test matrices

Verification

• git diff --check
• hugo --quiet
• rendered English and Chinese HTML routes are non-empty and contain the correctness-proof section
• typos check

The document continues to distinguish merged source and deployed documentation from the still-unpublished server release.

[cloudflare-workers-and-pages]

Deploying silo-pgsty-com with    Cloudflare Pages

Latest commit:	ef26b40
Status:	✅  Deploy successful!
Preview URL:	https://84de36cc.silo-pgsty-com.pages.dev
Branch Preview URL:	https://codex-expand-copyobject-chec.silo-pgsty-com.pages.dev

View logs