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_search/ST-SET-88.cbl

@@ -0,0 +1,24 @@
+      * ==== TYPE: ST-SET-88 ====
+      * FEATURE: SET 88-level condition to TRUE / FALSE
+      * STATEMENT: SET
+      * BRANCHES: 4, DECISIONS: 2
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. SET88.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-FLAG    PIC X.
+          88 WS-ACTIVE   VALUE 'Y'.
+          88 WS-INACTIVE VALUE 'N'.
+       PROCEDURE DIVISION.
+       MAIN.
+           SET WS-ACTIVE TO TRUE.
+           IF WS-ACTIVE
+               DISPLAY 'OK: SET TRUE'
+           ELSE
+               DISPLAY 'ERROR: SET TRUE'.
+           SET WS-ACTIVE TO FALSE.
+           IF WS-INACTIVE
+               DISPLAY 'OK: SET FALSE'
+           ELSE
+               DISPLAY 'ERROR: SET FALSE'.
+           STOP RUN.