feat(D.2b): UiChatView dat-font transcript + 1-line wheel quantum

- Add `DatFont` property (UiDatFont?): when set, OnDraw uses
  ctx.DrawStringDat + datFont.MeasureWidth for all transcript lines;
  BitmapFont path unchanged as fallback when DatFont is null.
- Cache `_lastDatFont` alongside `_lastFont` so HitChar hit-tests the
  same advance source that drew the last frame.
- HitChar prefers `_lastDatFont` (via UiDatFont.GlyphAdvance) over
  `_lastFont` (via bf.Advance) for column resolution, keeping
  drag-select and Ctrl+C accurate with the dat font.
- Scroll event handler uses DatFont?.LineHeight first, so the wheel
  quantum stays correct when the dat font has a different line height.
- WheelLines 3f → 1f: retail UIElement_Text::HandleMouseWheel
  (@0x471450) advances one line per notch; our 3-line quantum was
  wrong.
- Add UiChatViewDatFontTests: pins GlyphAdvance formula
  (Before+Width+After = 10) and CharIndexAt dat-advance integration.

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

tests/AcDream.App.Tests/UI/UiChatViewDatFontTests.cs

@@ -0,0 +1,30 @@
+using AcDream.App.UI;
+using DatReaderWriter.Types;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiChatViewDatFontTests
+{
+    // Synthetic per-char advance: each glyph 10px (Before=2,Width=6,After=2).
+    private static FontCharDesc Glyph(char c) => new()
+    {
+        Unicode = c, HorizontalOffsetBefore = 2, Width = 6, HorizontalOffsetAfter = 2,
+        OffsetX = 0, OffsetY = 0, Height = 12, VerticalOffsetBefore = 0,
+    };
+
+    [Fact]
+    public void CharIndexAt_UsesDatGlyphAdvance()
+    {
+        float Adv(char c) => UiDatFont.GlyphAdvance(Glyph(c));
+        Assert.Equal(0, UiChatView.CharIndexAt("abc", Adv, 4f));
+        Assert.Equal(1, UiChatView.CharIndexAt("abc", Adv, 12f));
+        Assert.Equal(3, UiChatView.CharIndexAt("abc", Adv, 100f));
+    }
+
+    [Fact]
+    public void GlyphAdvance_MatchesRetailFormula()
+    {
+        Assert.Equal(10f, UiDatFont.GlyphAdvance(Glyph('x')));
+    }
+}