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/core.py

@@ -211,11 +211,21 @@ class _BrParser:
                 seq.add(Assign(tgt, info))
                 self.advance()
                 # 跳过 READ 语句剩余行（AT END / NOT AT END / END-READ）
+                # 遇到新的语句关键词时停止，避免贪婪吞咽后续内容
+                _stmt_boundary = re.compile(
+                    r'^(IF |EVALUATE |PERFORM |SEARCH |INITIALIZE |STRING |'
+                    r'UNSTRING |CALL |ACCEPT |READ |WRITE |REWRITE |SET |'
+                    r'INSPECT |MOVE |COMPUTE |ADD |SUBTRACT |MULTIPLY |DIVIDE |'
+                    r'GO\s+TO |GOBACK |STOP\s+RUN|EXIT\s|CLOSE |OPEN |DISPLAY |'
+                    r'DELETE |START |'
+                    r'END-IF|END-PERFORM|END-EVALUATE|END-READ)', re.IGNORECASE)
                 while self.pos < len(self.lines):
                     cl = self.clean()
                     if cl in ('END-READ', 'END-READ.'):
                         self.advance()
                         break
+                    if _stmt_boundary.match(cl):
+                        break
                     self.advance()
                 continue
             m_set_false = re.match(r'^SET\s+(\w[\w-]*)\s+TO\s+FALSE\s*$', line, re.IGNORECASE)
@@ -658,8 +668,13 @@ class _BrParser:
         node = BrIf(cond_text)
         node.cond_tree = parse_compound_condition(node.condition, self.fields)
         node.true_seq = self.parse_seq(['ELSE', 'END-IF'])
-        if self.clean() == 'ELSE':
-            self.advance()
+        clean = self.clean()
+        if clean.startswith('ELSE'):
+            self.advance()  # consume ELSE keyword
+            rest = clean[4:].strip() if len(clean) > 4 else ''
+            # ELSE IF → reinsert IF statement as next line for recursive parse
+            if rest.upper().startswith('IF '):
+                self.lines.insert(self.pos, rest)
             node.false_seq = self.parse_seq(['END-IF'])
         if self.clean() == 'END-IF':
             self.advance()