fix: align multipart completion checksum errors

[Vonng]

Summary

• return BadDigest for CompleteMultipartUpload object checksum and explicit checksum-type mismatches
• compare FULL_OBJECT and COMPOSITE symmetrically, including the previously accepted composite-to-full direction
• treat omission of the optional x-amz-checksum-type header as no explicit type assertion instead of silently coercing it to COMPOSITE
• return InvalidRequest with the algorithm and actual part number when a composite completion omits a part checksum
• keep global hash.ChecksumMismatch, PutObject, UploadPart, algorithm-mismatch, and CRC64NVME canonicalization behavior unchanged
• keep all new error helpers and request-state markers package-private so the exported Go compatibility baseline does not change

Evidence and scope

AWS documents BadDigest for a completion checksum-type mismatch. The missing-part response is backed by the real S3 InvalidRequest transcript in boto/s3transfer#241 and its merged fix in boto/s3transfer#242.

The omitted-type change is a deliberate interoperability relaxation: this optional header is checked only when the client actually sends it. The previous path could reject a valid FULL_OBJECT completion while printing FULL_OBJECT as both the received and expected type.

This intentionally does not implement #50. The current CRC64NVME plus COMPOSITE canonicalization remains pinned until a raw AWS probe establishes whether S3 rejects or canonicalizes that request. The completion-side raw-token residue is recorded in the accompanying design document rather than claimed fixed here.

Validation

• git diff origin/main...HEAD --check
• CI-equivalent gofmt, CGO_ENABLED=0 go build ./..., and go vet ./...
• rebrand/exported-symbol compatibility guard and runtime delivery checks
• go test ./cmd ./internal/hash -count=1
  • cmd: 121.544s
  • internal/hash: 0.481s
• Claude Code claude-opus-5 adversarial acceptance at maximum effort: ACCEPT, high confidence, no blockers
  • reproduced the pre-fix composite-as-FULL_OBJECT 200/commit bypass with a go -overlay probe
  • confirmed every new assertion fails against the pre-fix implementation
  • probed all five checksum algorithms and both type directions

Fixes #48