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-EVAL-ALSO.cbl

@@ -0,0 +1,22 @@
+      * ==== TYPE: ST-EVAL-ALSO ====
+      * FEATURE: EVALUATE ALSO (multiple subjects)
+      * STATEMENT: EVALUATE ALSO
+      * BRANCHES: 4, DECISIONS: 1
+       IDENTIFICATION DIVISION.
+       PROGRAM-ID. EVLALS.
+       DATA DIVISION.
+       WORKING-STORAGE SECTION.
+       01 WS-STATUS  PIC X(1) VALUE 'A'.
+       01 WS-TYPE    PIC X(1) VALUE 'X'.
+       PROCEDURE DIVISION.
+       MAIN.
+           EVALUATE WS-STATUS ALSO WS-TYPE
+               WHEN 'A' ALSO 'X'
+                   DISPLAY 'OK: A-X'
+               WHEN 'A' ALSO 'Y'
+                   DISPLAY 'A-Y'
+               WHEN 'B' ALSO ANY
+                   DISPLAY 'B-ANY'
+               WHEN OTHER
+                   DISPLAY 'OTHER'.
+           STOP RUN.