[Rust] Edit with audit logging in fd5 crate

[gerchowl]

Parent: #161

Goal

Extend edit.rs to append an audit entry on every attribute edit.

Spec

• EditPlan gains: message: Option<String>, author: Author
• EditResult gains: audit_entry: AuditEntry
• EditPlan::apply() flow becomes:
  1. Read current content_hash → parent_hash
  2. Read current attribute value → old
  3. Write new attribute value
  4. Create AuditEntry
  5. Append to _fd5_audit_log
  6. Recompute content_hash
  7. Return EditResult with the entry

TDD

• Test edit creates audit entry with correct parent_hash
• Test edit preserves existing log entries
• Test author from identity is recorded
• Test message defaults to "Edit ."
• Test content_hash covers the audit log (tampering detected)