feat: add candidate ranker with confidence gate

worker/lyra_worker/ranker.py

@@ -0,0 +1,15 @@
+from dataclasses import replace
+
+from lyra_worker.confidence import score_confidence
+from lyra_worker.quality import rank_key
+from lyra_worker.types import Candidate, MBTarget
+
+
+def rank_candidates(
+    target: MBTarget, candidates: list[Candidate], min_confidence: float = 0.7
+) -> list[Candidate]:
+    """Score, gate by confidence, and sort best-first (quality, then confidence)."""
+    scored = [replace(c, confidence=score_confidence(target, c)) for c in candidates]
+    kept = [c for c in scored if c.confidence >= min_confidence]
+    kept.sort(key=lambda c: (rank_key(c), c.confidence), reverse=True)
+    return kept