feat(chat): Phase J Tier 1+2 - @ verb prefix, /retell, /framerate, /loc

Three-tier rollout per the 2026-04-25 retail @help dump showing the
full ACE command surface. Tier 1 + most of Tier 2 in one commit.

TIER 1 - @ as / equivalent

ACE accepts both / and @ as verb prefixes (per its own help text:
"Note: You may substitute a forward slash (/) for the at symbol
(@)."). ChatInputParser now normalises @ to / for the verb-match
phase and re-enters parsing. Critical: for verbs we don't recognise
(@acehelp, @tele, @die, @version, @loc-on-server, @nonsense, ...),
the original @ is kept in the message text so ACE's CommandManager
intercepts the message server-side. If we substituted / there too,
ACE would treat it as plain Talk and broadcast it.

Result: @a hi / @tell Bob hi / @help / @clear / @reply / @retell
all route exactly like their / counterparts. @acehelp / @tele /
@version / @die etc. pass through to the server intact.

TIER 2 - client-only commands

- /retell <msg> (also @retell): resend to the last person you
  tell'd. Mirrors retail @retell. ChatVM tracks
  LastOutgoingTellTarget on each OnSelfSent(Tell, ...) entry —
  SenderGuid==0 distinguishes outgoing echo from inbound whispers,
  same way LastIncomingTellSender already worked. ChatInputParser
  takes a new optional lastOutgoingTellTarget param.

- /framerate (also @framerate): prints "Framerate: 144.2 FPS"
  into chat. Wired via a new ChatVM.FpsProvider Func<float>
  callback set by GameWindow at construction (closes over
  _lastFps). Falls back to "(provider unavailable)" if no
  callback is wired (tests / pre-live).

- /loc (also @loc): prints "Location: (123.4, 567.8, 60.0)" into
  chat. Wired via ChatVM.PositionProvider Func<Vector3> closing
  over GetDebugPlayerPosition() in GameWindow. ACE has a server-
  side @loc too; client wins here (instantaneous + uses the local
  interpolated position).

ChatPanel.TryHandleClientCommand grew @ aliases for /help /clear
/framerate /loc and the new EqAny helper for case-insensitive
multi-string matching. Help text rewritten to reference the
/ <-> @ equivalence and point at @acehelp / @acecommands for ACE's
full command list.

TIER 3 - automatic (no code)

Most retail @-commands (@allegiance motd, @afk, @die, @lifestone,
@corpse, @marketplace, @pkarena, @emote/@emotes, @fillcomps,
@permit, @consent, @squelch, @unsquelch, @messagetypes, @age,
@birth, @day, @endurance, @pklite, @version, @filter, @unfilter,
@loadfile, @log, @marketplace, ...) are server-side ACE commands.
Tier 1's passthrough takes care of them automatically — they
arrive via Talk, ACE recognises the @ and intercepts, replies via
SystemChat (which our 0xF7E0 wiring renders as [System] lines).

DEFERRED

- @saveui / @loadui / @lockui: ImGui layout save/load, ~1 hr
  standalone task. Filed for follow-up.
- @title <text>: rename chat window. ImGui window-id complications.
- Toggle-style @framerate (FPS overlay on/off): print-once is
  simpler and matches retail's most-common usage.

30 new tests:
- ChatInputParserAtPrefixTests: 11 covering @-prefix recognition,
  unknown-@ passthrough, /retell and @retell.
- ChatVMRetellAndProvidersTests: 8 covering LastOutgoingTellTarget
  tracking, FpsProvider/PositionProvider callbacks, no-provider
  fallback.
- ChatPanelInputTests: +3 (/framerate, @loc, @acehelp passthrough).

Solution total: 1063 green (243 Core.Net + 160 UI + 660 Core),
0 warnings.

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

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

@@ -107,7 +107,11 @@ public sealed class ChatPanel : IPanel
                 return;
             }
 
-            var parsed = ChatInputParser.Parse(trimmed, ChatChannelKind.Say, _vm.LastIncomingTellSender);
+            var parsed = ChatInputParser.Parse(
+                trimmed,
+                ChatChannelKind.Say,
+                _vm.LastIncomingTellSender,
+                _vm.LastOutgoingTellTarget);
             if (parsed is { } p)
             {
                 ctx.Commands.Publish(new SendChatCmd(p.Channel, p.TargetName, p.Text));
@@ -154,21 +158,44 @@ public sealed class ChatPanel : IPanel
     {
         if (trimmed.Length == 0) return false;
 
-        if (trimmed.Equals("/help", StringComparison.OrdinalIgnoreCase) ||
-            trimmed.Equals("/?",    StringComparison.OrdinalIgnoreCase) ||
-            trimmed.Equals("/h",    StringComparison.OrdinalIgnoreCase))
+        // /help, /?, /h — also @help, @?, @h per ACE's "/ ↔ @" equivalence.
+        if (EqAny(trimmed, "/help", "/?", "/h", "@help", "@?", "@h"))
         {
             _vm.ShowSystemMessage(BuildHelpText());
             return true;
         }
 
-        if (trimmed.Equals("/clear", StringComparison.OrdinalIgnoreCase) ||
-            trimmed.Equals("/cls",   StringComparison.OrdinalIgnoreCase))
+        // /clear, /cls — also @clear, @cls.
+        if (EqAny(trimmed, "/clear", "/cls", "@clear", "@cls"))
         {
             _vm.Clear();
             return true;
         }
 
+        // /framerate — also @framerate. Prints current FPS to chat.
+        if (EqAny(trimmed, "/framerate", "@framerate"))
+        {
+            _vm.ShowFps();
+            return true;
+        }
+
+        // /loc — also @loc. Prints current player position to chat.
+        // ACE has a server-side @loc too; client-side wins here
+        // (instantaneous + uses our local interpolated position).
+        if (EqAny(trimmed, "/loc", "@loc"))
+        {
+            _vm.ShowLocation();
+            return true;
+        }
+
+        return false;
+    }
+
+    /// <summary>Case-insensitive multi-string equality test.</summary>
+    private static bool EqAny(string s, params string[] options)
+    {
+        for (int i = 0; i < options.Length; i++)
+            if (s.Equals(options[i], StringComparison.OrdinalIgnoreCase)) return true;
         return false;
     }
 
@@ -178,10 +205,11 @@ public sealed class ChatPanel : IPanel
     /// 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).";
+        "Note: / and @ are equivalent prefixes.\n" +
+        "Chat: /say (default), /tell <name>, /reply, /retell\n" +
+        "Channels: /general /trade /fellowship /allegiance\n" +
+        "          /patron /vassals /monarch /covassals\n" +
+        "          /lfg /roleplay /society /olthoi\n" +
+        "Client: /help (this) /clear /framerate /loc\n" +
+        "Server: type @acehelp or @acecommands for ACE's full list.";
 }