docs(ui): align CLAUDE.md + roadmap + memory with staged UI strategy

Landed the UI framework design in 2026-04-24-ui-framework.md yesterday;
this commit propagates the decisions across the documents that future
sessions touch first, so the three-layer pattern is discoverable without
re-reading the full plan.

Changes:

* NEW memory/project_ui_architecture.md — evergreen crib-sheet:
  three-layer diagram, AcDream.UI.Abstractions contract, D.2a/D.2b
  split, module layout, hard rules, why staged not pure-custom.

* CLAUDE.md: new paragraph describing the three-layer UI split, naming
  AcDream.UI.Abstractions as the plugin-facing contract, pointing at
  the full plan + memory crib.

* docs/architecture/acdream-architecture.md: new "UI Architecture"
  companion-stack diagram after Layer 0-5 (doesn't renumber the main
  stack), plus step 6a "UI tick" in Per-Frame Update Order.

* docs/plans/2026-04-11-roadmap.md Phase D tightened:
  - D.2 split explicitly into D.2a (Hexa.NET.ImGui scaffold + abstraction
    layer) and D.2b (custom retail-look backend, implements same contracts).
  - D.3 AcFont / D.4 dat sprites / D.7 cursor flagged as D.2b dependencies.
  - D.5 core panels / D.6 HUD flagged as abstraction-layer deliverables
    — ship with D.2a, reskinned by D.2b.
  - D.8 Sound marked superseded (shipped as Phase E.2).
  - F.5 core panels + H.1 chat-window cross-references updated to say
    they target AcDream.UI.Abstractions, unblocked by D.2a.
  - Shipped-phases table untouched.

* docs/research/retail-ui/00-master-synthesis.md: scope note at top
  clarifies the Keystone research is the D.2b (custom backend)
  foundation, NOT where D.2a starts.

* ~/.claude/.../memory/MEMORY.md: one-line index entry pointing at the
  new project_ui_architecture.md (so session auto-load surfaces it).

Zero code changes; doc-only. dotnet build stays green. All verification
greps pass (see plan file for exact checks).

docs/architecture/acdream-architecture.md

@@ -66,6 +66,32 @@ game state through well-defined interfaces that the retail client never had.
 └──────────────────────────────────────────────────────────────┘
 ```
 
+### UI Architecture (companion stack, spans Layers 1 & 5)
+
+The UI is split into its own three-layer stack with a swappable backend,
+designed 2026-04-24. Full design: `docs/plans/2026-04-24-ui-framework.md`.
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│  UI BACKEND (swappable)                                     │
+│  Hexa.NET.ImGui (Phase D.2a, short-term)                    │
+│   or custom retail-look toolkit (Phase D.2b, later)         │
+├─────────────────────────────────────────────────────────────┤
+│  AcDream.UI.Abstractions (stable contract)                  │
+│  ViewModels, Commands, IPanel, IPanelHost, IPanelRenderer   │
+│  ► plugin-facing UI API lives HERE, not in the backend      │
+├─────────────────────────────────────────────────────────────┤
+│  Game state + events (unchanged)                            │
+│  IGameState / IEvents / WorldSession — UI only reads        │
+└─────────────────────────────────────────────────────────────┘
+```
+
+The backend is pluggable; ViewModels / Commands / `IPanelRenderer` are
+stable across the swap. ImGui persists forever as the
+`ACDREAM_DEVTOOLS=1` devtools overlay regardless of which backend owns
+the game UI. See `memory/project_ui_architecture.md` for the session
+crib-sheet version.
+
 ---
 
 ## Project Structure (target)
@@ -206,6 +232,12 @@ exposes them as `WorldEntitySnapshot`. GameWindow becomes thin.
 
 6. Plugin tick
    └── Fire IEvents, drain IActions queue
+
+6a. UI tick
+    IPanelHost.Draw → iterate registered IPanel instances, build
+    ViewModels from IGameState, dispatch user Commands via ICommandBus.
+    Backend-agnostic — ImGui or custom retail-look draws here depending
+    on which is compiled in. See docs/plans/2026-04-24-ui-framework.md.
 ```
 
 ---