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_control/ST-GOTO-DEPEND.cbl

@@ -0,0 +1,25 @@
+      * ==== TYPE: ST-GOTO-DEPENDING ====
+      * FEATURE: GO TO ... DEPENDING ON
+      * STATEMENT: GO TO DEPENDING ON
+      * BRANCHES: 0, DECISIONS: 1
+      * NOTE: GO TO DEPENDING ON is parsed as pass-through (no IF branches)
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. GTODEP.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-SEL    PIC 9(1) VALUE 2.
+       PROCEDURE DIVISION.
+       MAIN.
+           GO TO PARA-1 PARA-2 PARA-3
+               DEPENDING ON WS-SEL.
+           DISPLAY 'FALL THROUGH'.
+           STOP RUN.
+       PARA-1.
+           DISPLAY 'PARA-1'.
+           STOP RUN.
+       PARA-2.
+           DISPLAY 'PARA-2'.
+           STOP RUN.
+       PARA-3.
+           DISPLAY 'PARA-3'.
+           STOP RUN.