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-CALL-VALUE.cbl

@@ -0,0 +1,20 @@
+      * ==== TYPE: ST-CALL-VALUE ====
+      * FEATURE: CALL ... BY VALUE with mixed mechanisms
+      * STATEMENT: CALL BY VALUE
+      * BRANCHES: 2, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. CALLVL.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-A PIC 9(5) VALUE 10.
+       01 WS-B PIC 9(5) VALUE 20.
+       PROCEDURE DIVISION.
+       MAIN.
+           CALL 'SUBPGM' USING
+               BY VALUE WS-A
+               BY REFERENCE WS-B.
+           IF WS-A = 10
+               DISPLAY 'OK: BY VALUE'
+           ELSE
+               DISPLAY 'ERROR: BY VALUE'.
+           STOP RUN.