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-COMPLEX.cbl

@@ -0,0 +1,26 @@
+      * ==== TYPE: ST-COMPLEX ====
+      * FEATURE: COMPUTE with multiple operators
+      * STATEMENT: COMPUTE
+      * BRANCHES: 4, DECISIONS: 2
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. COMPLX.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-X       PIC 9(5) VALUE 10.
+       01 WS-Y       PIC 9(5) VALUE 20.
+       01 WS-Z       PIC 9(5) VALUE 5.
+       01 WS-R1      PIC 9(5) VALUE 0.
+       01 WS-R2      PIC 9(5) VALUE 0.
+       PROCEDURE DIVISION.
+       MAIN.
+           COMPUTE WS-R1 = WS-X + WS-Y.
+           COMPUTE WS-R2 = (WS-Y - WS-X) * WS-Z.
+           IF WS-R1 = 30
+               DISPLAY 'OK: R1=30'
+           ELSE
+               DISPLAY 'ERROR: R1'.
+           IF WS-R2 = 50
+               DISPLAY 'OK: R2=50'
+           ELSE
+               DISPLAY 'ERROR: R2'.
+           STOP RUN.