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-IF-DEEP.cbl

@@ -0,0 +1,24 @@
+      * ==== TYPE: ST-IF-DEEP ====
+      * FEATURE: IF nested 3+ levels deep
+      * STATEMENT: IF (nested)
+      * BRANCHES: 6, DECISIONS: 3
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. IFDEEP.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-X       PIC 9(1) VALUE 1.
+       01 WS-Y       PIC 9(1) VALUE 2.
+       01 WS-Z       PIC 9(1) VALUE 3.
+       PROCEDURE DIVISION.
+       MAIN.
+           IF WS-X = 1
+               IF WS-Y = 2
+                   IF WS-Z = 3
+                       DISPLAY 'OK: NESTED'
+                   ELSE
+                       DISPLAY 'ERR: Z'
+               ELSE
+                   DISPLAY 'ERR: Y'
+           ELSE
+               DISPLAY 'ERR: X'.
+           STOP RUN.