feat(ui): AcDream.UI.ImGui backend — Hexa.NET.ImGui + Silk.NET input bridge

Second piece of Phase D.2a: the ImGui-specific backend that implements
AcDream.UI.Abstractions' IPanelRenderer / IPanelHost. No GameWindow
hookup yet — compiles standalone for clean review before integration.

Packages:
  * Hexa.NET.ImGui 2.2.9 (auto-generated from cimgui 1.92.2b)
  * Hexa.NET.ImGui.Backends 1.0.18 (consolidated — OpenGL3 is here)
  * Silk.NET.Input 2.23.0 + Silk.NET.OpenGL 2.23.0 (matches AcDream.App)

Files:

  ImGuiBootstrapper.cs
    One-shot static Initialize(glslVersion) / Shutdown() pair. Creates
    the ImGui context, applies dark style, enables NavEnableKeyboard,
    and boots ImGuiImplOpenGL3. Re-init is a no-op.

  SilkInputBridge.cs
    Event-driven Silk.NET -> ImGui IO bridge. Subscribes on construction;
    Dispose() unsubscribes. Covers:
      - KeyDown/Up -> ImGui.AddKeyEvent with modifier latching
        (Ctrl/Shift/Alt/Super routed via both ModXxx flags AND named
        key events so both IsKeyPressed checks and ImGui shortcut
        matching work)
      - KeyChar -> AddInputCharacter for text fields
      - MouseMove -> AddMousePosEvent
      - MouseDown/Up -> AddMouseButtonEvent (L=0, R=1, M=2)
      - Scroll -> AddMouseWheelEvent (both axes)
    Silk.NET.Input.Key -> ImGuiKey map covers WASD, arrows, modifiers,
    letters, digits, function keys. Unmapped keys silently ignored.
    BeginFrame(displaySize, dt) sets IO.DisplaySize + IO.DeltaTime.

  ImGuiPanelRenderer.cs
    IPanelRenderer impl — one-line wrappers on ImGui.Begin/End,
    TextUnformatted, SameLine, Separator, ProgressBar. The ONLY place
    Hexa.NET.ImGui types appear outside bootstrap/input plumbing. Panels
    still never import ImGui.

  ImGuiPanelHost.cs
    IPanelHost impl. Dictionary keyed by IPanel.Id for idempotent
    Register. RenderAll iterates visible panels and calls their Render.
    Does NOT call ImGui.NewFrame / ImGui.Render — ownership belongs to
    the caller (GameWindow) so GL state is explicit. Diagnostic `Count`
    property.

No behavior change yet; next commit wires this into GameWindow behind
ACDREAM_DEVTOOLS=1 and ships the first visible VitalsPanel.

AcDream.slnx

@@ -7,6 +7,7 @@
     <Project Path="src/AcDream.Plugin.Abstractions/AcDream.Plugin.Abstractions.csproj" />
     <Project Path="src/AcDream.Plugins.Smoke/AcDream.Plugins.Smoke.csproj" />
     <Project Path="src/AcDream.UI.Abstractions/AcDream.UI.Abstractions.csproj" />
+    <Project Path="src/AcDream.UI.ImGui/AcDream.UI.ImGui.csproj" />
   </Folder>
   <Folder Name="/tools/">
     <Project Path="tools/RetailTimeProbe/RetailTimeProbe.csproj" />