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>

src/AcDream.App/UI/UiChatView.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// Scrollable chat transcript for the retail-look chat window. Renders the
+/// lines from <see cref="LinesProvider"/> bottom-pinned (newest at the bottom,
+/// like retail) with mouse-wheel scrollback. Whole-line vertical clipping keeps
+/// text inside the window.
+///
+/// <para>
+/// This is the read-only foundation. A follow-up sub-step adds glScissor-based
+/// clipping + word-wrap, drag-to-select, and Ctrl+C copy (which needs the
+/// <see cref="UiElement.CapturesPointerDrag"/> opt-out so an interior drag
+/// selects text instead of moving the window).
+/// </para>
+/// </summary>
+public sealed class UiChatView : UiElement
+{
+    /// <summary>One display line: pre-formatted text + its colour.</summary>
+    public readonly record struct Line(string Text, Vector4 Color);
+
+    /// <summary>Provider of the lines to show, oldest-first. Polled each frame.</summary>
+    public Func<IReadOnlyList<Line>> LinesProvider { get; set; } = static () => Array.Empty<Line>();
+
+    /// <summary>Font for the transcript; falls back to the context default.</summary>
+    public BitmapFont? Font { get; set; }
+
+    /// <summary>Backing fill behind the text (retail chat is a dark translucent box).</summary>
+    public Vector4 BackgroundColor { get; set; } = new(0f, 0f, 0f, 0.35f);
+
+    /// <summary>Inner text inset from the view edges, px.</summary>
+    public float Padding { get; set; } = 4f;
+
+    // Pixels the transcript is scrolled UP from the newest line (0 = pinned to bottom).
+    private float _scroll;
+    private const float WheelLines = 3f;   // lines advanced per wheel notch
+
+    /// <summary>
+    /// Clamp a scroll offset to [0, max] where max = content-height - view-height
+    /// (never negative — when everything fits, scroll is pinned to 0). Exposed for tests.
+    /// </summary>
+    public static float ClampScroll(float scroll, float contentHeight, float viewHeight)
+    {
+        float max = Math.Max(0f, contentHeight - viewHeight);
+        if (scroll < 0f) return 0f;
+        return scroll > max ? max : scroll;
+    }
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        ctx.DrawRect(0, 0, Width, Height, BackgroundColor);
+
+        var font = Font ?? ctx.DefaultFont;
+        if (font is null) return;
+
+        var lines = LinesProvider();
+        if (lines.Count == 0) return;
+
+        float lh = font.LineHeight;
+        float top = Padding, bottom = Height - Padding;
+        float innerH = bottom - top;
+        float contentH = lines.Count * lh;
+        _scroll = ClampScroll(_scroll, contentH, innerH);
+
+        // Bottom-pin: with _scroll==0 the LAST line ends at `bottom`; scrolling up
+        // shifts the whole block down so older lines are revealed at the top.
+        float baseY = bottom - contentH + _scroll;
+        for (int i = 0; i < lines.Count; i++)
+        {
+            float y = baseY + i * lh;
+            if (y < top || y + lh > bottom) continue;   // whole-line vertical clip (no scissor yet)
+            ctx.DrawString(lines[i].Text, Padding, y, lines[i].Color, font);
+        }
+    }
+
+    public override bool OnEvent(in UiEvent e)
+    {
+        if (e.Type == UiEventType.Scroll)
+        {
+            float lh = Font?.LineHeight ?? 16f;
+            // Silk wheel +Y = scroll up = reveal older = shift content down = larger _scroll.
+            _scroll += e.Data0 * WheelLines * lh;   // re-clamped next OnDraw against live content
+            return true;
+        }
+        return false;
+    }
+}