feat(#1867): lift UI Considerations into plan-phase must_haves (LIFT-01)

Separate *-UI-SPEC.md glob (edge-coverage glob exclusion at :746 left intact
- Hyrum/D-08); a terse lift bullet reusing the ## Edge Coverage rule verbatim
(covered -> truths string, backstop -> flat scalar {statement, verification:
backstop}, unresolved -> assumption; no new verb - ADR-550 #1278/#1154); and a
no-silent-drop checklist line. Lift logic lives ONLY in the plan-phase
workflow, never gsd-planner.md (D-10, agent-size cap).

NOTE: this grows plan-phase.md +862B, over the #1168 PRE_PHASE6 ceiling (94519)
by ~802B, so the workflow-size gates are RED until #1852's lazy-split lands the
-21KB headroom. Deliberately does NOT raise the ceiling constant (would collide
with #1820/#1835's in-flight raise). #1867 sequences after #1852; rebase +
regenerate the size baseline then.

gsd-core/workflows/plan-phase.md

@@ -610,6 +610,10 @@ SKETCH_FINDINGS_PATH=$(ls ./.claude/skills/sketch-findings-*/SKILL.md 2>/dev/nul
 PHASE_DIR_FOR_SPEC=$(_gsd_field "$INIT" phase_dir)
 SPEC_FILE=$(ls "${PHASE_DIR_FOR_SPEC}"/*-SPEC.md 2>/dev/null | grep -Ev -- '-(AI|UI)-SPEC\.md$' | head -1)
 SPEC_PATH="${SPEC_FILE}"
+# Resolve the phase UI-SPEC separately (the glob above excludes -UI-SPEC.md); it carries the
+# ## UI Considerations section the planner lifts by the same rule as ## Edge Coverage (#1867).
+UI_SPEC_FILE=$(ls "${PHASE_DIR_FOR_SPEC}"/*-UI-SPEC.md 2>/dev/null | head -1)
+UI_SPEC_PATH="${UI_SPEC_FILE}"
 ```
 
 ## 7.5. Verify Nyquist Artifacts
@@ -808,6 +812,7 @@ Output consumed by /gsd:execute-phase. Plans need:
 - must_haves for goal-backward verification
 - If the SPEC has an `## Edge Coverage` section, lift every `covered` edge's acceptance criterion into `must_haves.truths` as a plain string, and every `backstop` edge **as a structured flat-scalar marker** — an object item `{ statement: <the check>, verification: backstop }`, NOT a prose note (the verifier branches deterministically on the `verification: backstop` field; a parenthetical is unparseable — the #1110 fragility). Use a flat scalar `verification:` continuation key, never a nested object (ADR-550 #1278). At verify time a `backstop` truth the verifier cannot confirm with explicit evidence abstains → `human_needed` (reason `insufficient_spec`), never a silent pass (#1154; see `references/honest-verifier.md`). `unresolved` edges are explicit assumptions — surface them in the plan, do not silently drop them. **Otherwise** (`EDGE_ABSENT`): apply the SAME lift to the fallback report `{COVERAGE}` (per §C of `references/specless-probe-fallback.md`); a SPEC-supplied section is never re-run.
 - If the SPEC has a `## Prohibitions` section, lift every resolved prohibition into the `must_haves.prohibitions:` sibling block (NOT `truths` — ADR-550 D3) with `statement`+`status`+`verification`, via the single `projectProhibitions` serializer (Hyrum — no second serializer); unresolved -> flagged assumptions, don't drop; never put a must-NOT under `truths`. **Otherwise** (`PROHIB_ABSENT`), author the recalled prohibitions into the SAME block via the SAME `projectProhibitions` contract but **descriptor-less** (no `check_*`) so each disposes flagged-unverified; never auto-dismiss. Section-level precedence + no-silent-drop equality apply (§C).
+- If a `-UI-SPEC.md` exists (resolved above as `UI_SPEC_PATH`) with a `## UI Considerations` section, lift it by the **identical rule** as `## Edge Coverage` above — `covered` → `must_haves.truths` string, `backstop` → flat scalar `{ statement, verification: backstop }`, `unresolved` → explicit planner assumption (no new verb — ADR-550 #1278/#1154; #1867). Read it from `UI_SPEC_PATH` (the SPEC glob excludes `-UI-SPEC.md`).
 - **"Artifacts this phase produces" section (MANDATORY)** — list every symbol this phase creates: decorators, classes, functions, CLI flags, struct/dataclass fields, new file paths. The plan-review-convergence source-grounding pass reads this section to exclude newly-created symbols from drift verification; omitting it causes new symbols to be flagged for acknowledgement.
 </downstream_consumer>
 
@@ -854,6 +859,7 @@ Every task MUST include these fields — they are NOT optional:
 - [ ] must_haves derived from phase goal
 - [ ] Every PLAN.md includes an "Artifacts this phase produces" section listing symbols created by this phase (decorators, classes, functions, CLI flags, struct/dataclass fields, new file paths)
 - [ ] Every SPEC ## Edge Coverage covered/backstop edge is represented in a plan's must_haves (no silent drops)
+- [ ] Every UI-SPEC ## UI Considerations covered/backstop consideration is represented in a plan's must_haves (no silent drops)
 - [ ] Every SPEC ## Prohibitions resolved item is represented in a plan's must_haves.prohibitions (no silent drops)
 </quality_gate>
 ```