fix: rule_signature conditions=None防御 + 0行表格覆盖率 + UT覆盖 - Closes #21

- step3 rule_signature: trigger.conditions=None 时使用 `or []` 防御
- step1 _quick_validate: total_rows=0 时行覆盖率设为 100% 而非 0%
- test_step1: 新增 TestHasSectionContent (10个) + TestQuickValidateEmptySections (2个)
- test_step3: 新增 TestRuleSignature (7个) + TestNormalizeRule (4个)

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

skills/ir_generation_skill/step1_semantic_index.py

@@ -566,6 +566,9 @@ def _quick_validate(
         if src.get("type") == "table" and src.get("row")
     )
     row_cov = covered_rows / max(total_rows, 1)
+    # When there are no table rows to cover, skip the check (not a coverage failure)
+    if total_rows == 0:
+        row_cov = 1.0
     print(f"  表格行覆盖率: {row_cov:.0%} ({covered_rows}/{total_rows} rows)", flush=True)
     if row_cov < SECTION_COVERAGE_TARGET:
         gaps["coverage_warnings"].append(