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-UNTIL.cbl

@@ -0,0 +1,21 @@
+      * ==== TYPE: ST-PERF-UNTIL ====
+      * FEATURE: PERFORM with UNTIL condition
+      * STATEMENT: PERFORM UNTIL
+      * BRANCHES: 2, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. PERFUN.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-COUNT  PIC 9(3) VALUE 0.
+       01 WS-TOTAL  PIC 9(5) VALUE 0.
+       PROCEDURE DIVISION.
+       MAIN.
+           PERFORM UNTIL WS-COUNT >= 5
+               ADD 10 TO WS-TOTAL
+               ADD 1 TO WS-COUNT
+           END-PERFORM.
+           IF WS-TOTAL = 50
+               DISPLAY 'OK: UNTIL LOOP'
+           ELSE
+               DISPLAY 'ERROR: UNTIL'.
+           STOP RUN.