feat(D.2b): scrollable retail chat window (read-only foundation)

Add UiChatView, a transcript widget for the retail-look UI: renders the
ChatVM tail bottom-pinned (newest at the bottom, like retail) with
mouse-wheel scrollback and whole-line vertical clipping so text stays
inside the frame. Hosted in a draggable/resizable UiNineSlicePanel and
wired into the UiHost next to the vitals window, fed by a dedicated
ChatVM (200-line tail) over the same live ChatLog. Per-ChatKind colour
palette (speech white, tells magenta, channels blue, system yellow,
emotes grey, combat orange).

This is the read-only foundation. The next sub-step adds glScissor
clipping + word-wrap, drag-to-select, and Ctrl+C copy -- the last needs
a CapturesPointerDrag opt-out on UiElement so an interior drag selects
text instead of moving the window (today an interior drag still moves
the window, same as the vitals panel).

Tests: UiChatView.ClampScroll (pin-to-bottom, cap-at-overflow,
never-negative).

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

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

@@ -0,0 +1,28 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiChatViewTests
+{
+    [Fact]
+    public void ClampScroll_PinsToZero_WhenContentFitsView()
+    {
+        // 5 lines of content in a taller view → nothing to scroll, pinned at 0.
+        Assert.Equal(0f, UiChatView.ClampScroll(50f, contentHeight: 80f, viewHeight: 200f));
+        Assert.Equal(0f, UiChatView.ClampScroll(0f, contentHeight: 80f, viewHeight: 200f));
+    }
+
+    [Fact]
+    public void ClampScroll_CapsAtContentMinusView_WhenOverflowing()
+    {
+        // Content 500, view 200 → max scrollback is 300px (oldest line at top).
+        Assert.Equal(300f, UiChatView.ClampScroll(1000f, contentHeight: 500f, viewHeight: 200f));
+        Assert.Equal(120f, UiChatView.ClampScroll(120f, contentHeight: 500f, viewHeight: 200f));
+    }
+
+    [Fact]
+    public void ClampScroll_NeverNegative()
+    {
+        Assert.Equal(0f, UiChatView.ClampScroll(-50f, contentHeight: 500f, viewHeight: 200f));
+    }
+}