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-VARY.cbl

@@ -0,0 +1,20 @@
+      * ==== TYPE: ST-PERF-VARY ====
+      * FEATURE: PERFORM VARYING ... FROM ... BY ... UNTIL
+      * STATEMENT: PERFORM VARYING
+      * BRANCHES: 0, DECISIONS: 0
+      * NOTE: PERFORM VARYING UNTIL is parsed but loop condition not IF branch;
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. PERFVA.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-I       PIC 9(3) VALUE 0.
+       01 WS-SUM     PIC 9(5) VALUE 0.
+       PROCEDURE DIVISION.
+       MAIN.
+           PERFORM VARYING WS-I FROM 1 BY 1 UNTIL WS-I > 5
+               ADD WS-I TO WS-SUM.
+           IF WS-SUM = 15
+               DISPLAY 'OK: 1+2+3+4+5=15'
+           ELSE
+               DISPLAY 'ERROR: SUM'.
+           STOP RUN.