Engine: extract COBOL data-division items and FD record layouts (the last structural datum the DB doesn't carry) #3246

Follow-up from the #3211 differential review. After #3200 (call graph) and #3201 (dataset lineage), the galaxy_ir SCOPE comment lists exactly one structural datum still owned by the forge tools and absent from the master DB:

NOT in the DB, so still owned by the forge tools: data-division items, FD record layouts, and reachability-based dead code.

(Reachability is a deliberate, separate design choice — usage_status is a by-name signal, see docs/unreferenced_by_name_contract.md. This issue is about the data half.)

Why it matters

Data-division items (the 01/05/... hierarchy: name, level, PIC, USAGE/COMP-3, OCCURS, REDEFINES, VALUE) and the FD record layouts bound to each file are the schema of the system. They are:

• what the Java forge must re-parse today to emit DTOs/entities (cobol_to_java_*, the DFHCOMMAREA→DTO / entity work cobol-to-java: 26 CBSA entities all map @Table(name = "DFHCOMMAREA") #3233/cobol-to-java: same-name programs collapse into one Service/Controller (the Java side of #3218) #3221) — it can't take these from the DB because the DB doesn't have them;
• required for field-level data-flow (which field flows into which dataset/copybook), the precision layer above Extract COBOL SELECT/ASSIGN and OPEN modes (and JCL DD bindings) by name, so dataset lineage can come from the engine #3201's file-level dataset lineage;
• the natural completion of the "consume engine signals" epic Epic: CICS Platform Modernization — consume engine signals in the refraction pipeline #3122 — once the DB carries record layouts, the refraction pipeline stops re-parsing COBOL entirely.

Scope sketch

• Extract the DATA DIVISION (WORKING-STORAGE, LINKAGE, FILE SECTION FD/01) as a nested item tree per program/copybook, into new *_data table(s) in the master DB, exposed via galaxy_ir (e.g. EngineFile.records / data_items).
• Handle the hard cases COBOL data division is made of: level numbers and nesting, OCCURS [DEPENDING ON], REDEFINES, PIC/USAGE COMP-3, VALUE, and copybook-supplied layouts.
• Add answer-key coverage (a records field) so the differential/scorer can grade it, same as COBOL modernization answer key: hand-verified per-program truth for the #3120 corpora, scored against forge and engine #3210 did for units/copybooks/lineage.

Acceptance

refraction_differential.py --corpus shows record layouts as a compared datum (not stated_absence via orphaned_vars), and the Java forge can source entity/DTO fields from the DB.

Part of epic #3122.