docs: record multipart checksum error contract

[Vonng]

Summary

• document the AWS evidence and upstream lineage behind CompleteMultipartUpload checksum failures return non-AWS error codes silo#48
• record the operation-scoped repair, hidden asymmetric type-check bypass, and complete regression matrix
• capture both initial Claude Code review rounds and the independent Opus 5 Max ACCEPT verdict
• disclose the algorithm-specific reach: CRC32/CRC32C type mismatches, earlier SHA1/SHA256 rejection, and the CRC64NVME completion-side residue
• define the raw AWS probe gate that keeps CRC64NVME + COMPOSITE is silently canonicalised to FULL_OBJECT instead of rejected silo#50 outside the repair
• provide matching English and Chinese design records

Related implementation

• fix: align multipart completion checksum errors silo#74

Validation

• git diff origin/main...HEAD --check
• make check
• Hugo strict build: 1,151 English and 1,151 Chinese pages
• internal link check: 466,092 rendered references across 1,323 HTML files
• Claude Code claude-opus-5 adversarial acceptance at maximum effort: ACCEPT, no blockers
• final EN/ZH parity review: no drift

This documentation PR does not close the server issue; the implementation PR owns that transition.

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