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>

src/AcDream.UI.Abstractions/Panels/Chat/ChatPanel.cs

@@ -94,7 +94,20 @@ public sealed class ChatPanel : IPanel
         if (renderer.InputTextSubmit("##chatinput", ref _input, InputBufferMaxLen, out var submitted)
             && submitted is not null)
         {
-            var parsed = ChatInputParser.Parse(submitted.Trim(), ChatChannelKind.Say, _vm.LastIncomingTellSender);
+            var trimmed = submitted.Trim();
+            // Phase J follow-up: client-side commands intercepted before
+            // the server-bound parse path. Avoids the /help round-trip
+            // that produced "Unknown command: help" duplicates from
+            // ACE's command-error replies, AND gives users a discoverable
+            // local cheat-sheet of acdream's own slash prefixes.
+            if (TryHandleClientCommand(trimmed))
+            {
+                _input = string.Empty;
+                renderer.End();
+                return;
+            }
+
+            var parsed = ChatInputParser.Parse(trimmed, ChatChannelKind.Say, _vm.LastIncomingTellSender);
             if (parsed is { } p)
             {
                 ctx.Commands.Publish(new SendChatCmd(p.Channel, p.TargetName, p.Text));
@@ -119,4 +132,56 @@ public sealed class ChatPanel : IPanel
         CombatLineKind.Error   => new Vector4(1.0f, 0.3f, 0.3f, 1.0f),
         _                      => new Vector4(1f, 1f, 1f, 1f),
     };
+
+    /// <summary>
+    /// Phase J follow-up: handle client-side slash commands before
+    /// the parser passes anything to the server bus. Returns true
+    /// when the input was consumed (and the caller should clear the
+    /// buffer + skip the SendChatCmd path); false otherwise.
+    /// </summary>
+    /// <remarks>
+    /// Recognised client-side commands:
+    /// <list type="bullet">
+    /// <item><c>/help</c>, <c>/?</c>, <c>/h</c> — render the slash-prefix
+    ///       cheat-sheet locally. Avoids the server's "Unknown command"
+    ///       round-trip when the user just wants to know what they can
+    ///       type.</item>
+    /// <item><c>/clear</c>, <c>/cls</c> — drain the chat log so the
+    ///       panel starts empty.</item>
+    /// </list>
+    /// </remarks>
+    private bool TryHandleClientCommand(string trimmed)
+    {
+        if (trimmed.Length == 0) return false;
+
+        if (trimmed.Equals("/help", StringComparison.OrdinalIgnoreCase) ||
+            trimmed.Equals("/?",    StringComparison.OrdinalIgnoreCase) ||
+            trimmed.Equals("/h",    StringComparison.OrdinalIgnoreCase))
+        {
+            _vm.ShowSystemMessage(BuildHelpText());
+            return true;
+        }
+
+        if (trimmed.Equals("/clear", StringComparison.OrdinalIgnoreCase) ||
+            trimmed.Equals("/cls",   StringComparison.OrdinalIgnoreCase))
+        {
+            _vm.Clear();
+            return true;
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Multi-line cheat-sheet text rendered by <c>/help</c>. ImGui's
+    /// <c>Text</c> path flows embedded newlines naturally so this lands
+    /// as one ChatLog entry that visually wraps to several lines.
+    /// </summary>
+    private static string BuildHelpText() =>
+        "acdream chat commands:\n" +
+        "  /say (default), /tell <name>, /reply, /general, /trade,\n" +
+        "  /fellowship, /allegiance, /patron, /vassals, /monarch,\n" +
+        "  /covassals, /lfg, /roleplay, /society, /olthoi\n" +
+        "  /help (this), /clear (clear chat tail)\n" +
+        "ACE server commands: prefix with @ (e.g. @acehelp, @acecommands).";
 }

src/AcDream.UI.Abstractions/Panels/Chat/ChatVM.cs

@@ -73,6 +73,18 @@ public sealed class ChatVM
         LastIncomingTellSender = entry.Sender;
     }
 
+    /// <summary>
+    /// Append a client-side system line to the chat log. Used by
+    /// client-handled commands (/help, /clear, future) to surface
+    /// local feedback without round-tripping the server.
+    /// </summary>
+    public void ShowSystemMessage(string text) => _log.OnSystemMessage(text, chatType: 0);
+
+    /// <summary>
+    /// Drain the chat log. Used by the /clear client-side command.
+    /// </summary>
+    public void Clear() => _log.Clear();
+
     /// <summary>
     /// Snapshot the tail of the chat log, formatted as display strings,
     /// oldest-first. Never returns null; returns an empty array if the