docs(#40): IMP-31 gate audit + activation reference cross-link

- u1: IMP-17-CARVE-OUT.md anchor cite :564 -> :570/:572/:575/:580/:664
- u2: new IMP-31-GATE-AUDIT.md (3-cond AND gate state + 8 issue-body axes)
- u3: backlog row + status-board section 5 cross-ref to audit doc (no verdict dup)

doc-only. no src/ templates/ tests/ touched. src/phase_z2_ai_fallback/ not created.

docs/architecture/IMP-17-CARVE-OUT.md

@@ -7,7 +7,7 @@
 - INSIGHT-MAP §3 — [`PHASE-Q-INSIGHT-TO-22STEP-MAP.md`](PHASE-Q-INSIGHT-TO-22STEP-MAP.md) (G3 AI repair fallback infra registry row, normal path = no).
 - 22-step pipeline — [`PHASE-Z-PIPELINE-OVERVIEW.md`](PHASE-Z-PIPELINE-OVERVIEW.md) Step 12 (lines 280-287), Step 16 (lines 318-325), Step 17 (lines 326-333).
 - Pattern shape reference (Phase Q Archive — link-only, do not port) — `src/content_editor.py:21,318` (httpx + retry shape, imports `sse_utils`) + `src/sse_utils.py:16-50` (SSE token parser).
-- Route hint emission site — `src/phase_z2_pipeline.py:564` (`restructure` → `ai_adaptation_required` route hint; deterministic emission today, AI consumer deferred to IMP-17).
+- Route hint surface (current anchors) — `src/phase_z2_pipeline.py:570` (conceptual comment), `:572` (`_IMP05_ROUTE_HINTS` table), `:575` (`restructure` → `ai_adaptation_required`), `:580` (`_imp05_route_hint`), `:664` (candidate_evidence emission). Deterministic emission today; AI consumer deferred to IMP-17 (this carve-out). Anchor pin: `tests/orchestrator_unit/test_imp17_comment_anchor.py`.
 
 ## Carve-out boundary