[Python] Chain verification in fd5 validate

[gerchowl]

Parent: #161

Goal

Extend fd5 validate to verify audit chain integrity alongside the Merkle tree.

Spec

• verify_chain(file: h5py.File) -> ChainStatus
• ChainStatus: Valid, NoLog, BrokenChain(index, expected, actual), Error(msg)
• Validation rules:
  1. Each entry's parent_hash must equal the previous entry's implicit new hash
  2. The first entry's parent_hash should be a valid sha256: prefixed hash
  3. No duplicate timestamps with identical changes
• Integrate into fd5 validate output: show "Audit chain: N entries, valid" or error
• verify() function already exists — add chain check as separate function, call from CLI

TDD

• Test valid chain passes
• Test tampered entry detected (modify middle entry)
• Test missing entry detected (gap in chain)
• Test file with no log returns NoLog
• Test single-entry chain