fix: step1 覆盖反馈重试增至 3 次 + 放宽质量门控 - Closes #75

- 重试次数 2→3，增加 LLM 补全机会
- 质量门控放宽：新增 sections 且无回归即采纳，不只严格要求覆盖率下降

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

skills/ir_generation_skill/step1_semantic_index.py

@@ -880,9 +880,9 @@ def run_ensemble_semantic_index(doc: dict) -> dict:
             if v:
                 print(f"    {k}: {len(v)} 个问题")
 
-        # Feedback retry: re-run with coverage feedback (up to 2 retries, quality-gated)
+        # Feedback retry: re-run with coverage feedback (up to 3 retries, quality-gated)
         retry_count = 0
-        while retry_count < 2:
+        while retry_count < 3:
             feedback = _build_coverage_feedback(gaps)
             if not feedback:
                 break
@@ -906,13 +906,16 @@ def run_ensemble_semantic_index(doc: dict) -> dict:
                             if src.get("section"):
                                 retry_sections.add(src["section"])
                     print(f"  重试新增 sections: {sorted(retry_sections)}", flush=True)
-                    # Quality gate: only include retry if it improves coverage
+                    # Quality gate: include retry if it adds new sections or doesn't regress coverage
                     trial_indices = semantic_indices + [retry_result]
                     trial_merged = ensemble_merge(trial_indices)
                     trial_passed, trial_gaps = _quick_validate(trial_merged, doc, all_paths)
                     trial_warnings = len(trial_gaps.get("coverage_warnings", []))
                     trial_missing = len(trial_gaps.get("missing_table_rows", []))
-                    if trial_warnings < pre_warnings or trial_missing < pre_missing_rows:
+                    improved = trial_warnings < pre_warnings or trial_missing < pre_missing_rows
+                    no_regression = trial_warnings <= pre_warnings and trial_missing <= pre_missing_rows
+                    has_new_sections = len(retry_sections) > 0
+                    if improved or (no_regression and has_new_sections):
                         semantic_indices.append(retry_result)
                         merged = trial_merged
                         passed, gaps = trial_passed, trial_gaps