COBOL units on DSF: ENVIRONMENT paragraphs, numeric names, forge headers (#3533)

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Engine (cobol func_start):

• SPECIAL-NAMES, I-O-CONTROL and REPOSITORY join the ENVIRONMENT DIVISION
  header shield: they were read as procedure units (navikt/DSF FRMERK; and
  18 SPECIAL-NAMES + 6 I-O-CONTROL phantom units across the crucible).
• An all-digit procedure name (DSF R001BYDL 0000. / 9999.) is a unit --
  at most 5 digits, so a 6/8-digit sequence field before a lone period
  (CardDemo 045100 .) never is; a numeric VALUE continuation is dropped by
  the existing cobol_sentence_start filter.

Forge:

• PROCEDURE DIVISION. (any blanks) is the header: new
  split_procedure_division() in the graveyard finder, used by the scorer's
  forge view; the DAG architect, schema forge and microservice slicer
  normalise the spacing the same way. DSF PLUKKFR/PLUKKFRN had no units.
• A header with its first statement on the same line (B. DISPLAY ...,
  DSF FO04D1X1) opens the unit, the statement its first body line.
• A conditional phrase (READ ... AT END GO TO X., INVALID KEY, SIZE ERROR,
  OVERFLOW, EXCEPTION) is conditional unless its END- verb closes it -- the
  key's CICS coverage: PL/I parity -- CICS call sites, handlers / UOW, data moves #3491 rule, now in the forge (else PLUKKFR's loop read dead).

Ledger: all 30 dsf_cobol_unit_extraction entries resolved (cause pruned);
R001BYDL's numeric units now surface the deliberate by-name divergence (0000
is the entry, 1000 only GO TOs itself) -> engine_dead_is_unreferenced_by_name.
Refraction: DSF full unexplained 18 -> 0. Golden masters: only the 24
phantom ENVIRONMENT units removed. Tri-comparison: cobol's validated
ctags-header class 4096 -> 4120 (ctags keeps reading those headers).

Closes #3533

Verification

• New cases: cobol func_start (0000. / 9999. valid; SPECIAL-NAMES., 045100 ., 01680012. invalid); forge test for spacing, inline header, AT END.
• Ledger clean (6 corpora); refraction --ci OK; golden masters PASS; ruff_audit --ci, mypy clean; full suite 11402 passed.

🤖 Generated with Claude Code

https://claude.ai/code/session_017ZsVaAkb86P5r5JXDC2Y9g

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