fix: Lark grammar + parse_file_section SD/ASCENDING KEY support
Bug fixes found through statement benchmark testing:
1. grammar.lark: Add ASCENDING/DESCENDING KEY IS + INDEXED BY to
occurs_clause — fixes HINA024 (SEARCH ALL) parsing crash
2. grammar.lark: Add SD (Sort Description) entry type to file_section
— fixes HINA034 (SORT), ST01, ST02 parsing crashes
3. read.py parse_file_section(): Handle SD blocks alongside FD blocks
— enables SORT/MERGE file structure extraction
4 previously crashing files now parse successfully:
- HINA024.cbl (SEARCH ALL): paras=3, files=0
- HINA034.cbl (SORT): paras=1, files=3
- ST01_SORT.cbl: paras=2, files=3
- ST02_MERGE.cbl: paras=1, files=4
Regression: 749 passed (unchanged — classify_program internally caught
the crashes, so tests already 'passed'; real improvement is in data
quality: structure extraction now works for these programs)
NB-076
dbee3b7251