feat: complete INSPECT/SEARCH support, fix PERFORM/EVAL coverage marking

- Add INSPECT (TALLYING/REPLACING/CONVERTING) with BEFORE/AFTER INITIAL
- Add SEARCH/SEARCH ALL with element-assignment path enumeration
- Fix _mark_perform compound condition marking via evaluate_tree
- Fix EVALUATE TRUE prior_false to collect all MC/DC false sets
- Add impossible path filtering (Pass A.5) with trace-to-root conflict detection
- Fix multi-line PERFORM VARYING parsing (VARYING/FROM/BY/UNTIL on separate lines)
- Remove dead code: agents.py LLM parser (replaced by rule-based _BrParser)
- 59 unit tests passing, 5 integration programs verified

cobol_testgen/models.py

@@ -115,6 +115,18 @@ class CondOr:
         self.right = right
 
 
+class BrSearch:
+    """SEARCH / SEARCH ALL 表查找"""
+    def __init__(self, table_name, is_all=False, varying=None):
+        self.table_name = table_name
+        self.is_all = is_all
+        self.varying = varying.upper() if varying else None
+        self.at_end_seq = BrSeq()
+        self.when_list = []       # [(condition_text, BrSeq)]
+        self.cond_trees = []      # [cond_tree, ...]
+        self.has_at_end = False
+
+
 class GoTo:
     """GO TO 节点：无条件跳转到指定段落"""
     def __init__(self, target: str, body_seq: 'BrSeq' = None):