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/UiElement.cs

@@ -102,6 +102,12 @@ public abstract class UiElement
     /// resizes it (window resize). Intended for top-level panels.</summary>
     public bool Resizable { get; set; }
 
+    /// <summary>If true, a left-drag starting on this element is delivered to the
+    /// element (e.g. text selection) instead of moving/resizing an ancestor window.
+    /// Edge resize on a resizable ancestor still wins — only the interior move /
+    /// drag-drop candidacy is suppressed in favour of the element's own handling.</summary>
+    public bool CapturesPointerDrag { get; set; }
+
     /// <summary>Minimum size enforced while resizing.</summary>
     public float MinWidth { get; set; } = 40f;
     public float MinHeight { get; set; } = 40f;