feat(D.2b): UI render infra — overlay layer, DrawFill, crisp text, write-mode focus

The retail-look render + focus primitives this chat pass builds on:

- TextRenderer: an OVERLAY layer (sprite/rect/text buckets flushed AFTER the
  normal layer) so an open popup composites on top of everything incl. rect
  panel backgrounds; a DrawFill primitive (solid quad via a 1x1 white texture)
  routed through the SPRITE bucket so a panel background draws UNDER its text
  instead of being washed by the later rect bucket; and the text pass now
  disables SampleAlphaToCoverage + Multisample so glyph alpha edges aren't
  dithered into MSAA coverage (the "fuzzy text") — self-contained GL state
  per feedback_render_self_contained_gl_state.
- UiRenderContext.DrawStringDat: snap the line baseline to a whole pixel ONCE
  then add the integer per-glyph offset (retail DrawCharacter takes an int
  pen-Y + schar m_VerticalOffsetBefore) — fixes the "letters dip down" jitter
  at a fractional line origin. Outline pass is now opt-in (retail gates it per
  element via SetOutline; default off = crisp fill-only). Adds DrawFill +
  Begin/EndOverlayLayer.
- UiElement: OnDrawOverlay + DrawOverlays (second traversal), FindRoot (blur
  self), ResetAnchorCapture (re-baseline an anchored element after reflow).
- UiRoot: runs the overlay pass after the main tree; Tab/Enter focuses the
  DefaultTextInput (write-mode activation); a left click on a non-edit target
  blurs the focused input (exit write mode without submitting).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiElement.cs

@@ -154,6 +154,16 @@ public abstract class UiElement
     /// </summary>
     protected virtual void OnDraw(UiRenderContext ctx) { }
 
+    /// <summary>
+    /// Draw content that must sit ON TOP of the ENTIRE UI, regardless of this
+    /// element's position in the tree — open menus, dropdowns, tooltips. Called in
+    /// a SECOND traversal after the whole tree's <see cref="OnDraw"/> pass, with the
+    /// same accumulated transform/alpha this element had during its normal draw.
+    /// Retail spawns popups as ROOT elements (UIElement_Menu::MakePopup) for exactly
+    /// this reason; this is the equivalent without reparenting. Default: nothing.
+    /// </summary>
+    protected virtual void OnDrawOverlay(UiRenderContext ctx) { }
+
     /// <summary>Per-frame tick (animations, timers, caret blink).</summary>
     protected virtual void OnTick(double deltaSeconds) { }
 
@@ -213,6 +223,34 @@ public abstract class UiElement
         }
     }
 
+    /// <summary>Second draw traversal: re-walks the tree applying the same
+    /// transform/alpha as <see cref="DrawSelfAndChildren"/> and calls
+    /// <see cref="OnDrawOverlay"/> on each element, so popups composite on top of
+    /// everything drawn in the main pass (dat-font glyphs and sprites share one
+    /// submission-ordered bucket, so later submissions win).</summary>
+    internal void DrawOverlays(UiRenderContext ctx)
+    {
+        if (!Visible) return;
+        ctx.PushTransform(Left, Top);
+        ctx.PushAlpha(Opacity);
+        try
+        {
+            OnDrawOverlay(ctx);
+            if (_children.Count > 0)
+            {
+                var ordered = _children.ToArray();
+                Array.Sort(ordered, static (a, b) => a.ZOrder.CompareTo(b.ZOrder));
+                for (int i = 0; i < ordered.Length; i++)
+                    ordered[i].DrawOverlays(ctx);
+            }
+        }
+        finally
+        {
+            ctx.PopAlpha();
+            ctx.PopTransform();
+        }
+    }
+
     internal void TickSelfAndChildren(double dt)
     {
         if (!Visible) return;
@@ -275,6 +313,22 @@ public abstract class UiElement
         Left = x; Top = y; Width = w; Height = h;
     }
 
+    /// <summary>Forget the captured anchor margins so the next <see cref="ApplyAnchor"/>
+    /// re-captures them from the CURRENT rect. Call after manually repositioning/resizing
+    /// an anchored element at runtime (e.g. reflowing the chat input when the channel
+    /// button width changes) so the new rect becomes the anchor baseline.</summary>
+    internal void ResetAnchorCapture() => _anchorCaptured = false;
+
+    /// <summary>Walk up to the owning <see cref="UiRoot"/> (the top of the tree), or null
+    /// if this element is not attached. Lets a widget reach focus/capture services — e.g.
+    /// a chat input blurring itself (exiting write mode) after submit.</summary>
+    internal UiRoot? FindRoot()
+    {
+        UiElement e = this;
+        while (e.Parent is not null) e = e.Parent;
+        return e as UiRoot;
+    }
+
     /// <summary>Compute an anchored child rect. Left&amp;Right ⇒ stretch width
     /// (keep both margins); Right only ⇒ pin to right at fixed width; otherwise
     /// pin left at fixed width. Same logic vertically.</summary>