feat(ui): Phase D.2a — VitalsPanel wired into GameWindow + backend pivot

Closes Phase D.2a. Launch with ACDREAM_DEVTOOLS=1 now shows a live
ImGui "Vitals" window whose HP bar reads CombatState.GetHealthPercent
for the local player. Without the env var the branches are dead code,
no ImGui context is created, and behaviour is identical to before.

GameWindow hunks:
  - fields: _imguiBootstrap / _panelHost / _vitalsVm + DevToolsEnabled
  - init (OnLoad): construct bootstrap + host, register VitalsPanel
  - GUID push: _vitalsVm?.SetLocalPlayerGuid(chosen.Id) at live-connect
  - frame begin: _imguiBootstrap.BeginFrame(dt) after GL clear
  - frame end: _panelHost.RenderAll(ctx) + _imguiBootstrap.Render() after debug overlay
  - input gating: skip WASD when ImGui.GetIO().WantCaptureKeyboard

Backend pivot: Hexa.NET.ImGui → ImGui.NET + Silk.NET.OpenGL.Extensions.ImGui.

First-light integration with the Hexa backend crashed 0xC0000005 inside
Hexa.NET.ImGui.Backends.OpenGL3.ImGuiImplOpenGL3.InitNative. Root cause:
Hexa's native OpenGL3 backend resolves GL function pointers via GLFW or
SDL internally; with Silk.NET (which uses neither) the pointers are null
and the native code crashes on first use. The mitigation path was
already planned — the design doc's Risk section called a pivot to
ImGui.NET a "one-morning operation" — and that's exactly what happened.

  - Packages: Hexa.NET.ImGui 2.2.9 + Hexa.NET.ImGui.Backends 1.0.18
    → ImGui.NET 1.91.6.1 + Silk.NET.OpenGL.Extensions.ImGui 2.23.0
  - ImGuiBootstrapper: was static Initialize(gl)+Shutdown() wrapping
    Hexa's OpenGL3 init; now an IDisposable wrapping Silk.NET's
    ImGuiController instance which handles GL backend init + input
    subscription in one go.
  - SilkInputBridge.cs deleted (~190 LOC): ImGuiController subscribes
    IKeyboard / IMouse events itself, we don't need a bespoke bridge.
  - ImGuiPanelRenderer: ImGuiNET.ImGui.* calls instead of
    Hexa.NET.ImGui.ImGui.*. Widget surface unchanged.

Boundary discipline is preserved — no panel imports ImGuiNET; only
ImGuiPanelRenderer does. The D.2b custom toolkit will implement the
same IPanelRenderer contract without touching panel code.

Out of scope (tracked for follow-up):
  - Stam/Mana currently return float? null (VitalsVM). Absolute values
    need LocalPlayerState + PlayerDescription (0x0013) parsing to be
    stored rather than discarded — filed as a post-D.2a issue.
  - Mouse-capture gating (WorldMouseFallThrough-style click-through
    tests) — not needed until we add clickable inventory items.

Roadmap + memory + architecture doc + UI framework plan updated in the
same commit per CLAUDE.md roadmap-discipline rules. 753 tests pass
(550 Core + 192 Core.Net + 11 new UI.Abstractions), 0 build warnings.

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

memory/project_ui_architecture.md

@@ -15,10 +15,14 @@
 └─────────────────────────────────────────┘
 ```
 
-- **UI backend** (bottom swap axis): `Hexa.NET.ImGui` for Phase D.2a
+- **UI backend** (bottom swap axis): **`ImGui.NET` + `Silk.NET.OpenGL.Extensions.ImGui`** for Phase D.2a
   (short-term, debugger-look, validates game logic fast). Custom
   retail-look toolkit for Phase D.2b (long-term, uses dat assets).
   ImGui stays **forever** as the `ACDREAM_DEVTOOLS=1` overlay.
+  (Pivoted from `Hexa.NET.ImGui` on 2026-04-25 — Hexa's native OpenGL3
+  backend resolves GL via GLFW/SDL internally and crashed `0xC0000005`
+  against Silk.NET; the Silk.NET extension is purpose-built for this
+  stack.)
 - **ViewModels + Commands** (the stable contract): per-panel data
   records (`VitalsVM`, `InventoryVM`, `ChatVM`, …) and action records
   (`UseItemCmd`, `SendChatCmd`, `CastSpellCmd`, …). Lives in
@@ -32,16 +36,19 @@
 - `src/AcDream.UI.Abstractions/` — `IPanel`, `IPanelHost`,
   `IPanelRenderer`, `ICommandBus`, all ViewModels + Commands. Backend-
   agnostic.
-- `src/AcDream.UI.ImGui/` — Hexa.NET.ImGui-based implementation of
-  `IPanelRenderer` + ImGui bootstrap. Phase D.2a.
+- `src/AcDream.UI.ImGui/` — `ImGui.NET` + `Silk.NET.OpenGL.Extensions.ImGui`
+  implementation of `IPanelRenderer` + ImGui bootstrap. Phase D.2a.
+  `ImGuiController` (from the Silk.NET extension) handles GL backend +
+  input event subscription; `ImGuiBootstrapper` is a thin IDisposable
+  wrapper; `ImGuiPanelRenderer` wraps the widgets `IPanelRenderer` needs.
 - `src/AcDream.UI.Retail/` (later) — custom retained-mode toolkit using
   dat assets, same `IPanelRenderer` contract. Phase D.2b.
 
 ## Hard rules
 
 1. **No panel references a backend namespace.** If a panel imports
-   `Hexa.NET.ImGui` or a custom-toolkit widget class directly, it's a
-   bug.
+   `ImGuiNET` / `Silk.NET.OpenGL.Extensions.ImGui` or a custom-toolkit
+   widget class directly, it's a bug — extend `IPanelRenderer` instead.
 2. **Plugin API targets the abstraction layer only.** Plugins define
    `IPanel` instances; they never see which backend draws them.
 3. **Features that only ImGui can express → not in `IPanelRenderer`.**