Replace the private, file-only mock AuditSink trait with the public async
cosmian_kms_interfaces::AuditSink defined in the previous commit:

- FileSink: AuditSink wraps the file backend (lock+recover retry loop moves
  from writer_supervisor into FileSink::resume(), never returns Err in
  practice — preserves the always-start/self-heal behavior exactly).
- write_failure_is_fatal() = false (preserves the file backend's historical
  log-and-continue behavior).
- Added AuditSink::is_write_capacity_exceeded() (default false) so the
  generic writer loop can skip a write without an error-per-event log spam;
  FileSink is the only implementor that ever returns true (max_size_bytes cap).
- writer_loop/write_draft_to_chain are now generic over any AuditSink and
  fully backend-agnostic (no file-specific cap/path knowledge left).
- Added AuditEventDraft::finalize() (cosmian_kms_access) to share the
  draft->AuditEvent+row_hash computation between the async steady-state
  writer and the sync recovery-time sentinel writer (torn-write/reanchor),
  which runs inside spawn_blocking with no async runtime available.
- AuditFileStore's public API (start/start_with_max_size/enqueue/flush/
  new_disconnected) is unchanged; no consumer (middleware, kms/mod.rs,
  tests) needed changes.

All 47 audit tests (access + core::audit + middleware + integration) green,
clippy clean, no behavior change observed in file-backend tests.