Closes audit N-02: docs/adr/ADR-F13-synthesis-engine.md was missing, blocking R-34. Documents spike result (GO, Opus APPROVE 2 passes), four-stage lead taxonomy, grounding discipline, three-level zero-fact-leak invariant, and falsification-gate in _parse_conjecture_output. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>