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_arithmetic/ST-ADD-ROUNDED.cbl

@@ -0,0 +1,20 @@
+      * ==== TYPE: ST-ADD-ROUNDED ====
+      * FEATURE: ADD ROUNDED
+      * STATEMENT: ADD ROUNDED
+      * BRANCHES: 2, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. ADDRND.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-VAL1    PIC 9(3)V99 VALUE 100.50.
+       01 WS-VAL2    PIC 9(3)V99 VALUE 200.75.
+       01 WS-RESULT  PIC 9(3)V99 VALUE 0.
+       01 WS-CHECK   PIC 9(3)V99 VALUE 301.25.
+       PROCEDURE DIVISION.
+       MAIN.
+           ADD WS-VAL1 TO WS-VAL2 GIVING WS-RESULT ROUNDED.
+           IF WS-RESULT = WS-CHECK
+               DISPLAY 'OK: 100.50+200.75=301.25'
+           ELSE
+               DISPLAY 'ERROR: WRONG SUM'.
+           STOP RUN.