fix: accept loader S3 checksum metadata

[pjy008008]

Summary

• accept the operating market-loader's content-sha256 S3 metadata key when verifying version-pinned source receipts
• keep the adapter's sha256 key canonical for new writes
• reject objects that publish both keys with conflicting hashes

Operational evidence

A four-resolution feature-backfill canary rejected four otherwise matching receipts with sha256 metadata mismatch. Read-only inspection confirmed that, for each exact S3 version, the catalog content_hash, byte size, version ID, SSE-S3 state, and S3 checksum all match; only the metadata key name differs (content-sha256).

TDD / validation

• red: loader metadata compatibility test failed before the fix
• 90 passed, 1 deselected, 4 subtests passed across storage adapter and feature-output trust-boundary tests
• Ruff passed
• mypy passed

Safety

The compatibility reader refuses contradictory sha256 and content-sha256 values, so the exact-version integrity boundary remains fail-closed.