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_move/ST-UNSTRING-BASIC.cbl

@@ -0,0 +1,23 @@
+      * ==== TYPE: ST-UNSTRING-BASIC ====
+      * FEATURE: UNSTRING space-delimited into multiple fields
+      * STATEMENT: UNSTRING
+      * BRANCHES: 0, DECISIONS: 0
+      * NOTE: UNSTRING is pass-through; no IF
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. UNSBAS.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-SRC     PIC X(20) VALUE 'ABC DEF GHI'.
+       01 WS-A       PIC X(5).
+       01 WS-B       PIC X(5).
+       01 WS-C       PIC X(5).
+       PROCEDURE DIVISION.
+       MAIN.
+           MOVE SPACES TO WS-A WS-B WS-C.
+           UNSTRING WS-SRC DELIMITED BY SPACES
+               INTO WS-A WS-B WS-C.
+           IF WS-A = 'ABC'
+               DISPLAY 'OK: UNSTRING'
+           ELSE
+               DISPLAY 'ERROR: UNSTRING'.
+           STOP RUN.