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_file/ST-WRITE-AFTER.cbl

@@ -0,0 +1,26 @@
+      * ==== TYPE: ST-WRITE-AFTER ====
+      * FEATURE: WRITE AFTER/BEFORE ADVANCING
+      * STATEMENT: WRITE AFTER
+      * BRANCHES: 0, DECISIONS: 0
+      * NOTE: WRITE AFTER is pass-through; no IF
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. WRTAFT.
+       ENVIRONMENT DIVISION.
+       INPUT-OUTPUT SECTION.
+       FILE-CONTROL.
+           SELECT OUT-FILE ASSIGN TO 'OUTDATA.DAT'.
+       DATA DIVISION.
+       FILE SECTION.
+       FD OUT-FILE.
+       01 OUT-REC PIC X(50).
+       WORKING-STORAGE SECTION.
+       01 WS-DATA   PIC X(50) VALUE 'TEST RECORD'.
+       PROCEDURE DIVISION.
+       MAIN.
+           OPEN OUTPUT OUT-FILE.
+           MOVE WS-DATA TO OUT-REC.
+           WRITE OUT-REC AFTER ADVANCING 1 LINE.
+           WRITE OUT-REC BEFORE ADVANCING 2 LINES.
+           CLOSE OUT-FILE.
+           DISPLAY 'OK: WRITE AFTER/BEFORE'.
+           STOP RUN.