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_inspect/ST-ACCEPT-DATE.cbl

@@ -0,0 +1,25 @@
+      * ==== TYPE: ST-ACCEPT-DATE ====
+      * FEATURE: ACCEPT FROM DATE / TIME / DAY
+      * STATEMENT: ACCEPT
+      * BRANCHES: 4, DECISIONS: 2
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. ACCDAT.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-DATE    PIC 9(8).
+       01 WS-TIME    PIC 9(8).
+       01 WS-DAY     PIC 9(5).
+       PROCEDURE DIVISION.
+       MAIN.
+           ACCEPT WS-DATE FROM DATE.
+           ACCEPT WS-TIME FROM TIME.
+           ACCEPT WS-DAY FROM DAY.
+           IF WS-DATE > 0
+               DISPLAY 'OK: DATE'
+           ELSE
+               DISPLAY 'ERROR: DATE'.
+           IF WS-TIME > 0
+               DISPLAY 'OK: TIME'
+           ELSE
+               DISPLAY 'ERROR: TIME'.
+           STOP RUN.