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_control/ST-IF-COMP.cbl

@@ -0,0 +1,22 @@
+      * ==== TYPE: ST-IF-COMP ====
+      * FEATURE: IF with compound conditions (AND / OR)
+      * STATEMENT: IF (compound)
+      * BRANCHES: 4, DECISIONS: 2
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. IFCOMP.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-A       PIC 9(5) VALUE 100.
+       01 WS-B       PIC 9(5) VALUE 200.
+       01 WS-C       PIC 9(5) VALUE 10.
+       PROCEDURE DIVISION.
+       MAIN.
+           IF WS-A > 50 AND WS-B > 100
+               DISPLAY 'OK: AND CONDITION'
+           ELSE
+               DISPLAY 'ERROR: AND'.
+           IF WS-A = 100 OR WS-C = 99
+               DISPLAY 'OK: OR CONDITION'
+           ELSE
+               DISPLAY 'ERROR: OR'.
+           STOP RUN.