fix: step3 过滤非法 source type + step1 重试质量门控 - Closes #57

- step3 _normalize_rule: 将 function_unit_description 等非法 source type 标准化为 text
- step1 覆盖反馈重试: 仅纳入实际提升覆盖率的 retry 结果，避免低质量输出稀释 ensemble
- 新增 UT: test_normalize_source_invalid_type

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

skills/ir_generation_skill/step3_merge_and_audit.py

@@ -170,8 +170,11 @@ def _normalize_rule(rule: dict) -> dict:
         }]
 
     # Ensure table/text sources have a section field (defensive against LLM omission)
+    # Also normalize invalid source types (LLM hallucinations like function_unit_description)
     sources = rule.get("sources", [])
     if sources:
+        valid_types = {"table", "text", "logic_tree"}
+
         # try to infer a default section from sibling sources or the rule path
         default_section = ""
         for s in sources:
@@ -186,6 +189,10 @@ def _normalize_rule(rule: dict) -> dict:
 
         for src in sources:
             stype = src.get("type", "")
+            # Normalize invalid source types to "text"
+            if stype and stype not in valid_types:
+                src["type"] = "text"
+                stype = "text"
             if stype in ("table", "text"):
                 if not src.get("section"):
                     src["section"] = default_section