feat(benchmark): Phase 2 (task 4) — actionable per-rule signals, ledger, FP knob

Complete Phase 2: make benchmark output drive per-rule improvement, wire the
loop-owned ledger, and turn a confirmed false positive into a score knob. Zero
new dependencies; detection stays byte-identical (recall 44/47 = 93.6%, FP 0%)
and the 90/10 gate holds.

- Per-rule attribution (4.1): BenchmarkMiss/BenchmarkFalsePositive carry rule_id;
  BenchmarkMetrics.per_rule aggregates matched/missed/false_positives per rule.
- Recommendation delta (4.2): evaluate_benchmark emits RuleRecommendation
  (keep/shadow/loosen) written to rule_policy_delta.json; the inert
  next_improvement_candidate stub is now rule-aware ("rule:<id>" or
  "language_hunter").
- Loop ledger (4.3): cli loads the ruleset with the rule_policy.json overlay
  (empty -> byte-identical; an entry overlays a rule's status/floors at load);
  read_rule_ledger/write_rule_ledger added for the evolve loop. A disabled rule
  in the ledger no longer fires (covered by an integration test).
- FP -> reachability knob (4.4): a confirmed false-positive learning lowers a
  finding's effective reachability multiplier in the score
  (fp_reachability_overrides + scoring reachability_override) instead of deleting
  the rule, so recall stays high while the score stops over-penalizing.

93 tests pass; ruff + mypy clean. Spec tasks 4.1-4.4 checked off — Phase 2
(Core: rules-as-data + benchmark signals) is complete. The loop-driven decision
to apply ledger flips remains the gated evolve loop (Phase 3/4).

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

src/openultrasast/ruleset/__init__.py

@@ -5,6 +5,8 @@ from .store import (
     PatternRule,
     RulesetError,
     load_ruleset,
+    read_rule_ledger,
+    write_rule_ledger,
     write_ruleset,
 )
 
@@ -13,5 +15,7 @@ __all__ = [
     "PatternRule",
     "RulesetError",
     "load_ruleset",
+    "read_rule_ledger",
+    "write_rule_ledger",
     "write_ruleset",
 ]

src/openultrasast/ruleset/store.py

@@ -81,6 +81,17 @@ def write_ruleset(path: Path, rules: Iterable[PatternRule]) -> None:
     path.write_text("\n\n".join(blocks) + "\n")
 
 
+def read_rule_ledger(path: Path) -> dict[str, dict[str, object]]:
+    """Read the loop-owned ``rule_policy.json`` overlay (status/floors per rule_id)."""
+    return _load_ledger(path)
+
+
+def write_rule_ledger(path: Path, entries: dict[str, dict[str, object]]) -> None:
+    """Write the loop-owned ``rule_policy.json`` overlay (used by the evolve loop)."""
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(json.dumps(entries, indent=2, sort_keys=True) + "\n")
+
+
 def _rule_from_dict(item: dict[str, object], path: Path) -> PatternRule:
     try:
         rule_id = str(item["rule_id"])