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.UI.Abstractions/Panels/Settings/SettingsPanel.cs

@@ -428,11 +428,40 @@ public sealed class SettingsPanel : IPanel
         if (renderer.SliderFloat("Font size (pt)", ref fontSize, 10f, 20f))
             _vm.SetChat(c with { FontSize = fontSize });
 
+        renderer.Spacing();
+        renderer.Text("Window transparency");
+        renderer.Separator();
+
+        // Campaign CH slice CH6c: retail's two linked opacity sliders
+        // (gmChatOptionsUI::InitOptions @0x0049FC60's DualHash pair). Dragging
+        // Background above Active drags Active UP to match; dragging Active below
+        // Background drags Background DOWN — ChatOpacityLink is the shared port of
+        // ChatInterface::SetDefaultOpacity/SetActiveOpacity (0x004F3BC0/0x004F3C40)
+        // that both this draft and the live RetailWindowOpacityController use, so the
+        // sliders track each other exactly like retail's options page.
+        float defaultOpacity = c.DefaultOpacity;
+        if (renderer.SliderFloat("Background opacity (unfocused)", ref defaultOpacity, 0f, 1f))
+        {
+            var (def, active) = ChatOpacityLink.SetDefault(c.ActiveOpacity, defaultOpacity);
+            _vm.SetChat(c with { DefaultOpacity = def, ActiveOpacity = active });
+        }
+
+        float activeOpacity = c.ActiveOpacity;
+        if (renderer.SliderFloat("Active opacity (typing / focused)", ref activeOpacity, 0f, 1f))
+        {
+            var (def, active) = ChatOpacityLink.SetActive(c.DefaultOpacity, activeOpacity);
+            _vm.SetChat(c with { DefaultOpacity = def, ActiveOpacity = active });
+        }
+
         renderer.Spacing();
         renderer.TextWrapped(
             "Channel filters hide messages from the chat window without "
-            + "changing your server-side subscriptions. Save persists; "
-            + "Cancel reverts.");
+            + "changing your server-side subscriptions. Window transparency "
+            + "applies to every retained window (chat, floaties, vitals, "
+            + "toolbar, inventory...) and fades whichever window doesn't "
+            + "currently have keyboard focus; Active opacity can never be "
+            + "lower than Background — dragging one past the other drags "
+            + "the other along. Save persists; Cancel reverts.");
     }
 
     /// <summary>