feat: add COBOL statement benchmark plan and 34 P0 sample programs

- docs/cobol-statement-benchmark-plan.md — full coverage matrix and gap analysis
- 34 P0 COBOL samples: arithmetic(9), move(5), file(6), control(6),
  inspect(3), search(2), perform(3)
- test-data/validate_statements.py — automatic validation script
- Validation: 34/34 samples pass preprocess + extract_structure

test-data/cobol/statement_perform/ST-PERF-TIMES.cbl

@@ -0,0 +1,19 @@
+      * ==== TYPE: ST-PERF-TIMES ====
+      * FEATURE: PERFORM ... TIMES
+      * STATEMENT: PERFORM TIMES
+      * BRANCHES: 2, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. PERFTM.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-COUNT  PIC 9(3) VALUE 0.
+       PROCEDURE DIVISION.
+       MAIN.
+           PERFORM 3 TIMES
+               ADD 1 TO WS-COUNT
+           END-PERFORM.
+           IF WS-COUNT = 3
+               DISPLAY 'OK: TIMES'
+           ELSE
+               DISPLAY 'ERROR: TIMES'.
+           STOP RUN.