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-STRING-DELIM.cbl

@@ -0,0 +1,26 @@
+      * ==== TYPE: ST-STRING-DELIM ====
+      * FEATURE: STRING with DELIMITED BY / SIZE
+      * STATEMENT: STRING
+      * BRANCHES: 0, DECISIONS: 0
+      * NOTE: STRING is pass-through; no IF
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. STRDEL.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-PART1   PIC X(5) VALUE 'ALPHA'.
+       01 WS-PART2   PIC X(4) VALUE 'BETA'.
+       01 WS-RESULT  PIC X(50).
+       01 WS-ptr     PIC 9(3) VALUE 1.
+       PROCEDURE DIVISION.
+       MAIN.
+           MOVE SPACES TO WS-RESULT.
+           MOVE 1 TO WS-ptr.
+           STRING WS-PART1 DELIMITED BY SPACES
+                  ',' DELIMITED BY SIZE
+                  WS-PART2 DELIMITED BY SPACES
+             INTO WS-RESULT WITH POINTER WS-ptr.
+           IF WS-RESULT(1:10) = 'ALPHA,BETA'
+               DISPLAY 'OK: STRING'
+           ELSE
+               DISPLAY 'ERROR: STRING'.
+           STOP RUN.