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>
