[Rust] Audit log data model and storage in fd5 crate

[gerchowl]

Parent: #161

Goal

Add audit module to the fd5 Rust crate with AuditEntry struct and read/write functions.

Spec

• AuditEntry struct (serde Serialize/Deserialize):
  • parent_hash: String
  • timestamp: String (ISO-8601)
  • author: Author (struct with author_type: String, id: String, name: String)
  • message: String
  • changes: Vec<Change> (struct with action: String, path: String, attr: String, old: Option<String>, new: Option<String>)
• read_audit_log(file: &File) -> Fd5Result<Vec<AuditEntry>> — read/parse _fd5_audit_log
• append_audit_entry(file: &File, entry: &AuditEntry) -> Fd5Result<()>
• JSON serialization via serde_json
• VarLenUnicode attribute storage

TDD

• Test round-trip: write → read → assert equal
• Test append preserves existing entries
• Test empty/missing log returns Ok(vec![])
• Test malformed JSON returns clear error
• Test serde serialization matches Python format exactly