feat(chat): Campaign CH slice CH6c — window opacity + transparency setting

Retail's ChatInterface::SetOpacity (0x004F3120) fades the WHOLE composited
window surface with one alpha; UiRenderContext.ApplyAlpha already gated
DrawSprite/DrawRect/DrawFill (since 1da697ec, pre-CH6) but DrawStringDat and
DrawString still passed applyAlpha:false, so text stayed sharp over a
translucent window. Both now route through the same chokepoint.

RetailWindowOpacityController (new) subscribes to a new
RetailWindowManager.WindowRegistered event and drives every registered
window's live Opacity from keyboard-focus state, applied to EVERY window
(chat, floaties, vitals, toolbar, ...) rather than retail's ChatInterface-only
scope — register row AP-190, retiring the stale AP-40 "fixed 0.75, no focus
transition" row in the same commit.

Verified retail's shipped opacity defaults from the decomp (constructor
literals, no cdb needed): the base ChatInterface ctor sets
DefaultOpacity=0.5/ActiveOpacity=1.0, kept unmodified by the four floating
windows; gmMainChatUI's own ctor overrides the main window to 1.0/1.0
(always fully opaque). acdream ships one shared global default (0.5/1.0)
rather than replicating the per-class override — also AP-190. The linking
invariant (raising default above active drags active UP; lowering active
below default drags default DOWN — never a clamp) is ported verbatim as
ChatOpacityLink in AcDream.UI.Abstractions, shared by the live controller
and the new Settings -> Chat tab's two linked opacity sliders.

Persistence: ChatSettings.DefaultOpacity/ActiveOpacity round-trip through
SettingsStore; Save pushes both through IRuntimeSettingsTargets.SetChatOpacity
into the live controller, no restart required.

Rider (CH6a/b re-review): strengthened the grip-media regression guard past
a bare SpriteFile != 0 check — ChatLayoutConformanceTests now drives each
live grip through a real UiRenderContext/TextRenderer (backed by the
in-memory RecordingGpuDevice test double) and asserts the draw call chain
actually queued sprite geometry, via a new TextRenderer.DebugSpriteSegments
test-only accessor.

Full Release suite 12,459 passed / 4 skipped / 0 failed (baseline
12,420/4/0). No subagents, no client launches (session hard constraints);
pending the next connected user gate for visual confirmation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AcDream.App/UI/RetailWindowManager.cs

@@ -33,6 +33,16 @@ public sealed class RetailWindowManager : IDisposable
     public IReadOnlyCollection<RetailWindowHandle> Windows => _byName.Values;
     public event Action<string, bool>? WindowVisibilityChanged;
 
+    /// <summary>
+    /// Fires once a NEW window finishes registering (not on a same-name/same-args
+    /// re-registration, which returns the existing handle early). Campaign CH slice
+    /// CH6c: <see cref="RetailWindowOpacityController"/> subscribes here so every
+    /// window this manager ever registers — chat, floaties, vitals, toolbar,
+    /// inventory, everything — picks up the live focus-driven opacity fade without
+    /// each individual <c>Mount*</c> call site needing to know about it.
+    /// </summary>
+    public event Action<RetailWindowHandle>? WindowRegistered;
+
     public RetailWindowHandle Register(
         string name,
         UiElement outerFrame,
@@ -79,6 +89,7 @@ public sealed class RetailWindowManager : IDisposable
         _byName.Add(name, handle);
         _byFrame.Add(outerFrame, handle);
         handle.NotifyInitialState();
+        WindowRegistered?.Invoke(handle);
         return handle;
     }