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.App/Rendering/GameWindow.cs

@@ -917,7 +917,15 @@ public sealed class GameWindow : IDisposable
                 // ChatPanel: reads the tail of the shared ChatLog. No GUID
                 // dependency — works pre-login (empty) and post-login (live
                 // tail of received speech/tells/channels/system msgs).
-                var chatVm = new AcDream.UI.Abstractions.Panels.Chat.ChatVM(Chat);
+                // FpsProvider + PositionProvider plumb the runtime state
+                // the client-side /framerate and /loc commands need; the
+                // panel asks the VM, the VM asks GameWindow via these
+                // delegates, no panel-vs-renderer-vs-state coupling.
+                var chatVm = new AcDream.UI.Abstractions.Panels.Chat.ChatVM(Chat)
+                {
+                    FpsProvider      = () => (float)_lastFps,
+                    PositionProvider = () => GetDebugPlayerPosition(),
+                };
                 _panelHost.Register(
                     new AcDream.UI.Abstractions.Panels.Chat.ChatPanel(chatVm));