fix(session): reject stale sqlite molt boundaries

[TZZheng]

Summary

• validate the selected SQLite psyche_molt boundary's fields_json.molt_count against the requested generation
• treat missing, malformed, boolean, or mismatched boundary metadata as a fast-path miss
• preserve the existing authoritative JSONL fallback and valid-boundary behavior

Validation

• PYTHONPATH=src python -m pytest -q tests/test_agent_session.py — 17 passed
• git diff --check canonical/main...HEAD — passed
• independent read-only review — PASS, no blockers

Risk

Invalid or stale sidecar metadata now takes the slower authoritative fallback instead of injecting totals from the wrong molt generation. The SQLite tier contract and generation-zero/no-boundary behavior remain intact.

Fixes #757