fix(chat): /help client-side handler + System dedup + ThatIsNotAValidCommand template

Phase J follow-up after a 2026-04-25 trace where typing /help
produced two identical "Unknown command: help" lines (ACE fires the
text via both GameMessageSystemChat 0xF7E0 and a paired
CommunicationTransientString 0x02EB), and the server's WeenieError
0x0026 trailer rendered cryptically as "WeenieError 0x0026".

Three small changes:

1. WeenieErrorMessages: add 0x0026 ThatIsNotAValidCommand ->
   "That is not a valid command." Plus 0x0414 / 0x050F that Phase J
   already added are now covered by tests too.

2. ChatLog.OnSystemMessage dedup. Track last system text + arrival
   time; if a second identical text shows up within 1 second,
   suppress. ACE's two-path send (gag warnings, command errors,
   etc.) collapses to a single chat line. Long bursts of repeated
   text still skip the duplicates without resetting the timer.

3. Client-side /help and /clear in ChatPanel. Intercepted BEFORE
   the parser passes to the server bus:
   - /help, /?, /h (case-insensitive) -> render local cheat-sheet
     listing acdream's slash prefixes via ChatLog.OnSystemMessage.
     Avoids the round-trip to ACE that produced the duplicate
     "Unknown command: help" lines AND gives users discoverability.
   - /clear, /cls -> drains the chat log so the panel starts empty.

   New ChatVM.ShowSystemMessage() + ChatVM.Clear() expose the
   minimum surface the panel needs to dispatch client-only feedback
   without coupling the panel to ChatLog directly.

12 new tests:
- 3 WeenieErrorMessages template adds (0x0026 / 0x0414 / 0x050F).
- 4 ChatLog dedup cases (immediate dup, different text, triplet,
  bookended-by-different-text).
- 5 ChatPanel client-command cases (/help, 3 alias variants,
  /clear).

Solution total: 1033 green (243 Core.Net + 130 UI + 660 Core),
0 warnings.

Acceptance: type /help in chat -> local help banner appears, no
server round-trip, no "Unknown command: help" duplicates. Type
/clear -> chat tail empty. Welcome banner + WeenieError-templated
"You are not in an allegiance!" / "You do not belong to a
Fellowship." continue rendering once each.

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

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

@@ -18,6 +18,75 @@ public sealed class ChatPanelInputTests
         public void Publish<T>(T command) where T : notnull => Published.Add(command);
     }
 
+    [Fact]
+    public void Submit_HelpCommand_RendersLocalHelpAndDoesNotPublish()
+    {
+        // Phase J follow-up: client-side commands (/help, /?, /h) are
+        // intercepted before the parser. They render a local cheat-sheet
+        // via ChatLog.OnSystemMessage and do NOT round-trip the server
+        // — that's what prevented the "Unknown command: help" duplicate
+        // ACE was firing back.
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = "/help",
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Empty(bus.Published);
+        var entry = Assert.Single(log.Snapshot());
+        Assert.Equal(ChatKind.System, entry.Kind);
+        Assert.Contains("acdream chat commands:", entry.Text);
+        Assert.Contains("/tell", entry.Text);
+    }
+
+    [Theory]
+    [InlineData("/?")]
+    [InlineData("/h")]
+    [InlineData("/HELP")]
+    public void Submit_HelpAliases_AlsoRenderLocalHelp(string raw)
+    {
+        var log = new ChatLog();
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = raw,
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Empty(bus.Published);
+        Assert.Single(log.Snapshot());
+    }
+
+    [Fact]
+    public void Submit_ClearCommand_DrainsLog_AndDoesNotPublish()
+    {
+        var log = new ChatLog();
+        log.OnSystemMessage("seed line", chatType: 0);
+        var vm  = new ChatVM(log);
+        var panel = new ChatPanel(vm);
+        var bus = new RecordingBus();
+        var renderer = new FakePanelRenderer
+        {
+            InputTextSubmitNextSubmitted = "/clear",
+            InputTextSubmitNextBufferAfter = "",
+        };
+
+        panel.Render(new PanelContext(0.016f, bus), renderer);
+
+        Assert.Empty(bus.Published);
+        Assert.Empty(log.Snapshot());
+    }
+
     [Fact]
     public void Submit_PlainText_PublishesSayCommand()
     {