@ feat(lab): wire completeness floor into consolidated v5 arm (gen-6)

T1 coding_markdown_v5 now serves CODING_MARKDOWN_TRIAGE_FLOOR_LAB_SYSTEM_PROMPT
= triage prompt + the verbatim LEAN_BATCH completeness clause (one internal
check for issues/parties/dates/numbers/exceptions/conflicts before drafting).
The floor joins the consolidated arm per "levers accrete, analysis ablates";
the completeness_floor receipt flag is the ablation boundary.

Wiring is sha-gate-safe by construction:
- chat.ts triage branch consults COMPLETENESS_FLOOR_ENABLED and serves the
  floor variant when set; the receipt sha covers systemPrompt + inventory.
- armExpectedSurface returns the same floor const, so the preflight/conformance
  sha expectation matches the served prompt exactly.
- conformance: completenessFloorCoding = arm === coding_markdown_v5 replaces
  the hard completeness_floor !== false; T2 (reqecho) stays floor-off so the
  echo contrast is unconfounded.

Verified: tsc --noEmit clean; preflight-only for coding_markdown_v5 registers
with a new floor-wired sha; floor const is exactly base + COMPLETENESS_FLOOR_BLOCK
(243 chars) and differs from the base prompt sha.

Co-Authored-By: Claude <noreply@anthropic.com>
@