fix: 3 critical parsing bugs found through statement benchmark testing

Bug 1: ELSE IF breaks IF false_seq parsing (core.py)
- _parse_if checked self.clean() == 'ELSE' which fails on 'ELSE IF ...'
- Fix: use startswith('ELSE'), reinsert IF portion for recursive parse
- Impact: ALL ELSE IF chains were silently dropped (huge branch loss)

Bug 2: READ skip loop greedily consumes subsequent statements (core.py)
- READ's AT END / NOT AT END skip loop used bare advance() with no
  statement boundary detection
- Fix: add _stmt_boundary regex that stops on IF/PERFORM/READ/etc.
- Impact: everything after first READ was consumed as 'AT END' lines

Bug 3: _walk() in extract_structure doesn't descend into BrPerform (__init__.py)
- Branch counting _walk() only handled BrIf/BrEval/BrSeq
- IF statements inside PERFORM bodies were never counted
- Fix: add BrPerform.body_seq and BrSearch descent

Combined impact: matching programs (MT01-33) now correctly report
their branches instead of 0. Full regression: 749 passed (unchanged).

cobol_testgen/__init__.py

@@ -372,7 +372,7 @@ def extract_structure(cobol_source: str) -> dict:
     file_sec = parse_file_section(preprocessed)
     open_dir = scan_open_statements(proc_div) if proc_div else {}
 
-    from .models import BrIf, BrEval, BrSeq, BrPerform, Assign, CondAnd, CondOr
+    from .models import BrIf, BrEval, BrSeq, BrPerform, BrSearch, Assign, CondAnd, CondOr
 
     decision_points = []
     total_branches = 0
@@ -403,6 +403,12 @@ def extract_structure(cobol_source: str) -> dict:
         elif isinstance(node, BrSeq):
             for child in node.children:
                 _walk(child, counter)
+        elif isinstance(node, BrPerform):
+            _walk(node.body_seq, counter)
+        elif isinstance(node, BrSearch):
+            _walk(node.at_end_seq, counter)
+            for _, seq in node.when_list:
+                _walk(seq, counter)
 
     if branch_tree:
         _walk(branch_tree, [0])