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.Core/Chat/ChatLog.cs

@@ -26,6 +26,17 @@ public sealed class ChatLog
     private readonly int _maxEntries;
     private uint _localPlayerGuid;
 
+    // Phase J follow-up: ACE often sends the same system text via two
+    // wire paths (GameMessageSystemChat 0xF7E0 + GameEventCommunication-
+    // TransientString 0x02EB) for back-compat — we wired both to
+    // OnSystemMessage in I.5/J, so the user saw lines like "Unknown
+    // command: help" twice. Dedupe within a short window: track the
+    // last system text + arrival time; if a second identical text
+    // shows up within one second, skip.
+    private string _lastSystemText = "";
+    private DateTime _lastSystemAt = DateTime.MinValue;
+    private static readonly TimeSpan SystemDedupWindow = TimeSpan.FromSeconds(1);
+
     public ChatLog(int maxEntries = 500)
     {
         if (maxEntries < 1) throw new ArgumentOutOfRangeException(nameof(maxEntries));
@@ -178,9 +189,28 @@ public sealed class ChatLog
             ChannelId: 0));
     }
 
-    /// <summary>GameEvent CommunicationTransientString (0x02EB) — e.g. "Your spell fizzled!"</summary>
+    /// <summary>
+    /// System chat — covers GameMessageSystemChat (0xF7E0
+    /// ServerMessage) and GameEventCommunicationTransientString
+    /// (0x02EB). Phase J follow-up: dedupe identical text arriving
+    /// within <see cref="SystemDedupWindow"/> so flows that fire on
+    /// both opcodes (e.g. "Unknown command: help" via help-command
+    /// failure path) only show once.
+    /// </summary>
     public void OnSystemMessage(string text, uint chatType)
     {
+        var now = DateTime.UtcNow;
+        if (text == _lastSystemText && (now - _lastSystemAt) < SystemDedupWindow)
+        {
+            // Suppress the dup — the wire-level duplicate isn't a
+            // user-meaningful signal. Reset the timer so a long burst
+            // of the same text still skips.
+            _lastSystemAt = now;
+            return;
+        }
+        _lastSystemText = text;
+        _lastSystemAt = now;
+
         Append(new ChatEntry(
             Kind: ChatKind.System,
             Sender: "",

src/AcDream.Core/Chat/WeenieErrorMessages.cs

@@ -68,6 +68,9 @@ public static class WeenieErrorMessages
     /// </summary>
     private static readonly Dictionary<uint, string> NoParamTemplates = new()
     {
+        // Command parser
+        [0x0026] = "That is not a valid command.",              // ThatIsNotAValidCommand
+
         // Tell-related
         [0x052B] = "That person is not available now.",         // CharacterNotAvailable