feat(fusion): Phase 13 — two-panel fusion adjudication

OpenUltraCode-style deepening for findings that need more reasoning than the
ranker→hunter→verifier→mapping loop provides.

- fusion.py (zero-dep core + lazy LLM path): a deterministic trigger policy
  (critical/high severity, verifier disagreement, static-vs-semantic conflict,
  gates-risky-fix, high-assurance request); two panels that steel-man the
  vulnerability and false-positive cases; a deterministic, auditable decider that
  reconciles to one of five dispositions (accepted/rejected/mitigated/deferred/
  blocked); full disclosure (votes, model IDs, degradations, warnings, decision
  source). fuse_findings_dispatch routes panels through the configured HarnessX
  provider when panel_model + extra are present, else deterministic + a degradation.
- config [fusion]: enabled (default true), panel_model, decider_model,
  high_assurance.
- cli: standard-mode fusion stage runs on triggered findings, writes fusion.json
  and a manifest `fusion` block; quick mode never fuses. reports.write_manifest
  gained the fusion summary param.
- 8.4 zero-dep guard extended to cover fusion.py (lazy harnessx import only).
- README fusion section marked implemented + documented; openrouter-sast-harness
  Phase 13 checked off.

tests/test_fusion.py (16 tests) + config tests. 169 passed, 4 skipped; ruff + mypy
clean; detection gate PASS.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/openultrasast/reports.py

@@ -73,6 +73,7 @@ def write_manifest(
     path: Path,
     score: dict[str, object] | None = None,
     degradations: list[dict[str, object]] | None = None,
+    fusion: list[dict[str, object]] | None = None,
 ) -> None:
     path.parent.mkdir(parents=True, exist_ok=True)
     verification_by_id = _verification_by_id(verifications)
@@ -99,6 +100,8 @@ def write_manifest(
         payload["score"] = score
     if degradations is not None:
         payload["degradations"] = degradations
+    if fusion is not None:
+        payload["fusion"] = fusion
     path.write_text(json.dumps(payload, indent=2, sort_keys=True) + "\n")