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_arithmetic/ST-MUL-BY.cbl

@@ -0,0 +1,19 @@
+      * ==== TYPE: ST-MUL-BY ====
+      * FEATURE: MULTIPLY ... BY (constant)
+      * STATEMENT: MULTIPLY
+      * BRANCHES: 2, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. MULBY.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-AMOUNT  PIC 9(5) VALUE 50.
+       01 WS-RESULT  PIC 9(5) VALUE 0.
+       PROCEDURE DIVISION.
+       MAIN.
+           MULTIPLY 3 BY WS-AMOUNT.
+           MOVE WS-AMOUNT TO WS-RESULT.
+           IF WS-RESULT = 150
+               DISPLAY 'OK: 50*3=150'
+           ELSE
+               DISPLAY 'ERROR'.
+           STOP RUN.