feat(D.2b): chat text selection + Ctrl-C copy

Windows-like selection in the retail chat window: left-click-drag selects
characters, Ctrl-C copies, Ctrl-A selects all. The selected span paints a
translucent highlight behind the text.

- UiElement.CapturesPointerDrag: a per-element opt-out so an interior drag is
  delivered to the widget (text selection) instead of moving/resizing the host
  window. UiRoot.OnMouseDown honours it AFTER edge-resize (a resizable window
  is still resizable from its frame) and BEFORE window-move.
- UiChatView: AcceptsFocus + IsEditControl + CapturesPointerDrag; caches the
  OnDraw layout so OnEvent hit-tests the same geometry; HitChar maps a local
  point to (line,col) with glyph-midpoint caret snapping; SelectedText joins a
  multi-line span with \n; Ctrl-C writes to IKeyboard.ClipboardText (only when
  non-empty, so an empty copy never clobbers the clipboard).
- UiHost exposes the wired IKeyboard (clipboard + Ctrl modifier state).

Adversarial-review fix (the 99 tests would have stayed green without it): a
coordinate-frame mismatch between MouseDown and MouseMove. UiRoot.OnMouseDown
dispatched HitTestTopDown's coords, which are relative to the TOP-LEVEL child,
while MouseMove/MouseUp use target.ScreenPosition. For the chat view inset at
(8,8) inside its window the anchor landed ~8px off the click. OnMouseDown now
delivers target-LOCAL coords like the other mouse events. Added a UiRoot
regression test asserting MouseDown and MouseMove share the target-local frame
for a nested child.

Decomp ref: SurfaceWindow text/selection model; clipboard via Silk.NET
IKeyboard.ClipboardText. Built with the chat-select-copy implement->review
workflow.

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

src/AcDream.App/UI/UiHost.cs

@@ -39,6 +39,13 @@ public sealed class UiHost : System.IDisposable
     public UiRoot Root { get; } = new();
     public TextRenderer TextRenderer { get; }
     public BitmapFont? DefaultFont { get; set; }
+
+    /// <summary>The last wired keyboard. Exposed so widgets that need clipboard
+    /// access (<see cref="IKeyboard.ClipboardText"/>) or modifier-key state
+    /// (<see cref="IKeyboard.IsKeyPressed"/>) — e.g. <see cref="UiChatView"/>'s
+    /// Ctrl+C copy — can reach the device. One-keyboard desktop: last wins.</summary>
+    public IKeyboard? Keyboard { get; private set; }
+
     private long _startTicks = System.Environment.TickCount64;
 
     public UiHost(GL gl, string shaderDir, BitmapFont? defaultFont = null)
@@ -82,6 +89,7 @@ public sealed class UiHost : System.IDisposable
 
     public void WireKeyboard(IKeyboard kb)
     {
+        Keyboard = kb;   // last wired keyboard wins (one-keyboard desktop)
         kb.KeyDown += (_, k, _) => Root.OnKeyDown((int)k);
         kb.KeyUp   += (_, k, _) => Root.OnKeyUp((int)k);
         kb.KeyChar += (_, c) => Root.OnChar(c);