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-READ-AT-END.cbl

@@ -0,0 +1,30 @@
+      * ==== TYPE: ST-READ-AT-END ====
+      * FEATURE: READ with AT END / NOT AT END
+      * STATEMENT: READ AT END
+      * BRANCHES: 0, DECISIONS: 0
+      * NOTE: READ AT END is pass-through; no IF
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. READAE.
+       ENVIRONMENT DIVISION.
+       INPUT-OUTPUT SECTION.
+       FILE-CONTROL.
+           SELECT IN-FILE ASSIGN TO 'INDATA.DAT'.
+       DATA DIVISION.
+       FILE SECTION.
+       FD IN-FILE.
+       01 IN-REC PIC X(80).
+       WORKING-STORAGE SECTION.
+       01 WS-STATUS PIC X VALUE 'N'.
+       01 WS-DATA   PIC X(80).
+       PROCEDURE DIVISION.
+       MAIN.
+           OPEN INPUT IN-FILE.
+           READ IN-FILE INTO WS-DATA
+               AT END MOVE 'Y' TO WS-STATUS
+               NOT AT END MOVE 'N' TO WS-STATUS.
+           IF WS-STATUS = 'Y'
+               DISPLAY 'OK: AT END REACHED'
+           ELSE
+               DISPLAY 'OK: DATA READ'.
+           CLOSE IN-FILE.
+           STOP RUN.