[Rust] Chain verification in fd5 crate

[gerchowl]

Parent: #161

Goal

Extend verify.rs to validate audit chain integrity.

Spec

• ChainStatus enum: Valid(usize) (entry count), NoLog, BrokenChain { index, expected, actual }, Error(String)
• verify_chain(file: &File) -> Fd5Result<ChainStatus>
• Integrate into Fd5Status or return separately
• Validation:
  1. Walk entries in order
  2. Each entry\u2019s parent_hash must form a valid chain
  3. No gaps or tampered entries

TDD

• Test valid chain → Valid(N)
• Test tampered middle entry → BrokenChain
• Test no log → NoLog
• Test single entry chain
• Test content_hash mismatch with chain (Merkle tree catches audit log tampering)