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-ADD-TO.cbl

@@ -0,0 +1,21 @@
+      * ==== TYPE: ST-ADD-TO ====
+      * FEATURE: ADD x TO y (constant / variable)
+      * STATEMENT: ADD
+      * BRANCHES: 2, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. ADDTO.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-VALUE   PIC 9(5) VALUE 100.
+       01 WS-RESULT  PIC 9(5) VALUE 0.
+       01 WS-DELTA   PIC 9(5) VALUE 25.
+       PROCEDURE DIVISION.
+       MAIN.
+           ADD 50 TO WS-VALUE.
+           MOVE WS-VALUE TO WS-RESULT.
+           ADD WS-DELTA TO WS-RESULT.
+           IF WS-RESULT = 175
+               DISPLAY 'OK: 100+50+25=175'
+           ELSE
+               DISPLAY 'ERROR: WRONG VALUE'.
+           STOP RUN.