feat(D.2b): UiChatInput — editable field, caret, 100-entry history (UIElement_Text port)

Ports retail UIElement_Text editable one-line mode (caret = glyph index;
caret pixel-X = sum of glyph advances via UiDatFont) and ChatInterface's
100-entry command history (up/down arrow; sentinel -1 = live line).
Submit (Enter/KeypadEnter) fires OnSubmit callback, clears, pushes history.
Draws via DrawStringDat (dat font) or DrawString (BitmapFont) fallback.
AcceptsFocus=true + IsEditControl=true so UiRoot routes Char/KeyDown to it
and suppresses global hotkeys while typing. 6 new tests, all green.

Decomp refs: UIElement_Text::MoveCursor @0x468d00,
             UIElement_Text::FindPixelsFromPos @0x472b40,
             ChatInterface::ProcessCommand @0x4f5100

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

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

@@ -0,0 +1,72 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiChatInputTests
+{
+    [Fact]
+    public void InsertChar_AdvancesCaret()
+    {
+        var input = new UiChatInput();
+        input.InsertChar('h'); input.InsertChar('i');
+        Assert.Equal("hi", input.Text);
+        Assert.Equal(2, input.CaretPos);
+    }
+
+    [Fact]
+    public void Backspace_DeletesBeforeCaret()
+    {
+        var input = new UiChatInput();
+        foreach (var c in "abc") input.InsertChar(c);
+        input.MoveCaret(-1);
+        input.Backspace();
+        Assert.Equal("ac", input.Text);
+        Assert.Equal(1, input.CaretPos);
+    }
+
+    [Fact]
+    public void Submit_FiresCallback_ClearsText_PushesHistory()
+    {
+        string? sent = null;
+        var input = new UiChatInput { OnSubmit = t => sent = t };
+        foreach (var c in "hello") input.InsertChar(c);
+        input.Submit();
+        Assert.Equal("hello", sent);
+        Assert.Equal("", input.Text);
+        Assert.Equal(0, input.CaretPos);
+    }
+
+    [Fact]
+    public void EmptySubmit_DoesNotFire()
+    {
+        int n = 0;
+        var input = new UiChatInput { OnSubmit = _ => n++ };
+        input.Submit();
+        Assert.Equal(0, n);
+    }
+
+    [Fact]
+    public void History_UpDownBrowsesPreviousSubmissions()
+    {
+        var input = new UiChatInput { OnSubmit = _ => {} };
+        foreach (var c in "first") input.InsertChar(c); input.Submit();
+        foreach (var c in "second") input.InsertChar(c); input.Submit();
+        input.HistoryPrev();
+        Assert.Equal("second", input.Text);
+        input.HistoryPrev();
+        Assert.Equal("first", input.Text);
+        input.HistoryNext();
+        Assert.Equal("second", input.Text);
+        input.HistoryNext();
+        Assert.Equal("", input.Text);
+    }
+
+    [Fact]
+    public void History_CapsAt100()
+    {
+        var input = new UiChatInput { OnSubmit = _ => {} };
+        for (int i = 0; i < 150; i++) { input.InsertChar('x'); input.Submit(); }
+        Assert.True(input.HistoryCount <= 100);
+    }
+}