docs(IMP-19): zone ratio reference + cross-link -- documented-axis close

Stage 5 commit for IMP-19 (gitea #19) — docs-only, no runtime surface.

- new: docs/architecture/IMP-19-ZONE-RATIO-REFERENCE.md (header + A1 consumer
  + A2 producer + A3 Phase Z solver delta + A4 IMP-09 boundary + A5
  re-activation gate / GR1-GR4).
- update: PHASE-Z-IMPLEMENTATION-ISSUE-BACKLOG.md — IMP-19 row pending ->
  documented + reference doc link; IMP-09 row carries soft back-link to the
  IMP-19 reference doc.
- update: PHASE-Q-INSIGHT-TO-22STEP-MAP.md §3 I4 row — prepend IMP-19 anchor
  + reference doc link (step/classification preserved).

Guardrails (Stage 1/2 binding contract): src/ untouched, no role-based
["배경","본심"] hardcoding into Phase Z, IMP-09 solver ownership preserved,
soft-link integrity holds. IMP-19 remains dormant until the A5 gate fires.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture/PHASE-Q-INSIGHT-TO-22STEP-MAP.md

@@ -122,7 +122,7 @@
 | B-2 verification 보조 | Step 1, 2, 14, 21, 22 | §2.7 H3 (text 추출 / 정규화 / 비교 utility) | pending | yes (UI/backend) |
 | IMP-17 AI repair fallback infra (carve-out — see [`IMP-17-CARVE-OUT.md`](IMP-17-CARVE-OUT.md)) | Step 12, 16, 17 | §2.6 G3 (`httpx` + SSE streaming + retry + JSON parse pattern) | pending | no (AI fallback only) |
 | I3 SVG 좌표 보강 | Step 0, 9 | §2.8 I3 (`renderer._preprocess_svg_data`) | pending | yes (deterministic) |
-| I4 zone 비중 분배 | Step 8 | §2.8 I4 (`renderer._group_blocks_by_area`) | pending | yes (deterministic) |
+| IMP-19 I4 zone 비중 분배 (reference — see [`IMP-19-ZONE-RATIO-REFERENCE.md`](IMP-19-ZONE-RATIO-REFERENCE.md)) | Step 8 | §2.8 I4 (`renderer._group_blocks_by_area`) | pending | yes (deterministic) |
 | H2 frame contract validation | Step 10 | §2.7 H2 (`content_verifier.verify_structure` pattern) | pending | yes (deterministic) |
 
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