feat(ui): #17 ChatPanel input field + slash commands + reply-to-last-tell

ChatPanel gains an Enter-to-submit input field via the I.1
InputTextSubmit widget. Submitted text routes through ChatInputParser
to a SendChatCmd published on ctx.Commands; LiveCommandBus (I.3)
handles the wire send + ChatLog echo.

Recognised prefixes (ported from holtburger commands.rs):

  /say msg or no prefix  -> Say
  /t Name msg or /tell   -> Tell  (first whitespace token = target)
  /r msg                 -> Tell  (target = LastIncomingTellSender)
  /g msg                 -> General
  /f msg                 -> Fellowship
  /a msg                 -> Allegiance
  /m msg                 -> Monarch
  /p msg                 -> Patron
  /v msg                 -> Vassals
  /cv msg                -> CoVassals
  /lfg msg               -> Lfg
  /trade msg             -> Trade
  /role msg              -> Roleplay
  /society msg           -> Society
  /olthoi msg            -> Olthoi

Edge cases: empty / whitespace / cmd-without-message / /r without
prior tell -> null (no-op). Unknown /xyz prefix -> Say with literal
text (matches holtburger's Talk(command) default arm).

ChatVM.LastIncomingTellSender populated only on incoming Tell entries;
discriminated by SenderGuid != 0 (OnSelfSent echoes always carry 0).

32 new tests:
- ChatInputParserTests: 22 covering every prefix + edge case
- ChatVMLastTellSenderTests: 6 covering capture + skip rules
- ChatPanelInputTests: 6 using FakePanelRenderer + recording
  ICommandBus to assert publish behaviour

UI.Abstractions.Tests: 60 -> 92. Solution total: 934 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.UI.Abstractions.Tests/Panels/Chat/ChatPanelInputTests.cs

@@ -0,0 +1,139 @@
+using AcDream.Core.Chat;
+using AcDream.UI.Abstractions.Panels.Chat;
+
+namespace AcDream.UI.Abstractions.Tests.Panels.Chat;
+
+/// <summary>
+/// Phase I.4: when the user submits text via the chat input field, the
+/// panel must publish a <see cref="SendChatCmd"/> to the command bus.
+/// We exercise the full Render path with the <see cref="FakePanelRenderer"/>
+/// pre-loading a "submitted" string and a recording bus capturing the
+/// resulting command.
+/// </summary>
+public sealed class ChatPanelInputTests
+{
+    private sealed class RecordingBus : ICommandBus
+    {
+        public List<object> Published { get; } = new();
+        public void Publish<T>(T command) where T : notnull => Published.Add(command);
+    }
+
+    [Fact]
+    public void Submit_PlainText_PublishesSayCommand()
+    {
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = "hello world",
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        var cmd = Assert.Single(bus.Published);
+        var sendCmd = Assert.IsType<SendChatCmd>(cmd);
+        Assert.Equal(ChatChannelKind.Say, sendCmd.Channel);
+        Assert.Null(sendCmd.TargetName);
+        Assert.Equal("hello world", sendCmd.Text);
+    }
+
+    [Fact]
+    public void Submit_TellSlashCommand_PublishesTellCommand()
+    {
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = "/t Bestie ping",
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        var sendCmd = Assert.IsType<SendChatCmd>(Assert.Single(bus.Published));
+        Assert.Equal(ChatChannelKind.Tell, sendCmd.Channel);
+        Assert.Equal("Bestie", sendCmd.TargetName);
+        Assert.Equal("ping", sendCmd.Text);
+    }
+
+    [Fact]
+    public void Submit_ReplySlashCommand_UsesLastIncomingTellSender()
+    {
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        log.OnTellReceived("Bestie", "ping", senderGuid: 0x5000_00AAu);
+
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = "/r back at you",
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        var sendCmd = Assert.IsType<SendChatCmd>(Assert.Single(bus.Published));
+        Assert.Equal(ChatChannelKind.Tell, sendCmd.Channel);
+        Assert.Equal("Bestie", sendCmd.TargetName);
+        Assert.Equal("back at you", sendCmd.Text);
+    }
+
+    [Fact]
+    public void Submit_EmptyOrWhitespace_PublishesNothing()
+    {
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = "   ",
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Empty(bus.Published);
+    }
+
+    [Fact]
+    public void NoSubmit_PublishesNothing()
+    {
+        // Most frames: user is typing or idle; submitted == null.
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = null,
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Empty(bus.Published);
+    }
+
+    [Fact]
+    public void Render_AlwaysCallsInputTextSubmit_ToShowTheField()
+    {
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = null,
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Contains(renderer.Calls, c => c.Method == "InputTextSubmit");
+    }
+}