Add local state write shadow validation

[huangruiteng]

Summary

• add preview-only shadow validation for local state write correctness packets
• detect stale active-state revisions and competing todo lease holders without mutating state
• cover clean, revision_conflict, and lease_conflict cases in the todo write correctness smoke

Validation

• python3 examples/todo-write-correctness-smoke.py
• python3 examples/local-state-write-correctness-contract-smoke.py
• python3 examples/refresh-state-write-correctness-smoke.py
• python3 -m py_compile loopx/local_state_write_correctness.py examples/todo-write-correctness-smoke.py
• git diff --check

Boundary

• public/private scan only matched existing public-boundary field names and imports; no private paths, raw logs, trajectories, or secrets were added

Review note: this is intentionally a dry-run/shadow fixture only. It does not enforce writes or change runtime mutation behavior.