fix(bitrot): reject trailing shard data

[cxymds]

Related Issues

Fixes rustfs/backlog#1518.

Summary of Changes

• Probe one byte past the expected streaming bitrot payload after all existing length and hash checks.
• Reject a valid encoded prefix followed by any trailing data.
• Add regressions for one-byte and multi-byte suffixes in the core verifier and PUT self-verification through inline and disk readers.
• Preserve valid, short-read, size-mismatch, hash-mismatch, streaming HighwayHash, and legacy whole-file behavior.

Verification

• rustfmt --edition 2024 --check crates/ecstore/src/erasure/coding/bitrot.rs crates/ecstore/src/set_disk/ops/bitrot_self_verify.rs
• git diff --check
• High-risk seven-role adversarial review completed with no unresolved findings.
• Full repository CI is required before this draft is marked ready.

Impact

PUT post-write verification now validates the complete reader rather than only the expected-length prefix. Each verification adds one single-byte EOF probe and no second full scan.

Additional Notes

The MinIO streaming layout, encoded-size formula, legacy format, and on-disk representation are unchanged.

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