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-REWRITE-FROM.cbl

@@ -0,0 +1,27 @@
+      * ==== TYPE: ST-REWRITE-FROM ====
+      * FEATURE: REWRITE ... FROM
+      * STATEMENT: REWRITE FROM
+      * BRANCHES: 0, DECISIONS: 0
+      * NOTE: REWRITE FROM is pass-through; no IF
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. REWFRM.
+       ENVIRONMENT DIVISION.
+       INPUT-OUTPUT SECTION.
+       FILE-CONTROL.
+           SELECT FILE-A ASSIGN TO 'FILEA.DAT'
+               ORGANIZATION IS INDEXED
+               ACCESS IS RANDOM.
+       DATA DIVISION.
+       FILE SECTION.
+       FD FILE-A.
+       01 REC-A PIC X(80).
+       WORKING-STORAGE SECTION.
+       01 WS-UPD    PIC X(80) VALUE 'UPDATED'.
+       PROCEDURE DIVISION.
+       MAIN.
+           OPEN I-O FILE-A.
+           MOVE WS-UPD TO REC-A.
+           REWRITE REC-A FROM WS-UPD.
+           CLOSE FILE-A.
+           DISPLAY 'OK: REWRITE FROM'.
+           STOP RUN.