feat(net): Phase F.1 GameEvent (0xF7B0) envelope dispatcher

Implements the inbound GameEvent routing layer — the single biggest
network-protocol gap per r08 (94 sub-opcodes, zero handled before).
WorldSession now detects 0xF7B0, parses the 16-byte header (guid +
gameEventSequence + eventType), and forwards to a pluggable
GameEventDispatcher.

Added:
- GameEventEnvelope record + TryParse with layout from
  ACE GameEventMessage.cs.
- GameEventType enum: all 94 S→C sub-opcodes from
  ACE.Server.Network.GameEvent.GameEventType, named per ACE conventions.
- GameEventDispatcher: handler registry + unhandled-counts bag for
  diagnostics ("which server events are firing that we don't parse?").
  Handlers invoked synchronously on the decode thread; thrown exceptions
  are swallowed + logged to stderr so one bad handler can't take down
  the packet loop.
- GameEvents parsers: ChannelBroadcast, Tell, TransientMessage,
  PopupString, WeenieError (+ WithString), UpdateHealth, PingResponse,
  MagicUpdateSpell. Each returns a typed record or null on malformed
  payload. String16L helper matches the existing CharacterList pattern
  (u16 length + ASCII bytes + 4-byte pad).
- WorldSession.GameEvents property exposing the dispatcher so
  GameWindow / UI / chat can register handlers at startup.

Wired into WorldSession.ProcessDatagram: new `else if (op ==
GameEventEnvelope.Opcode)` branch with TryParse + Dispatch.

Tests (13 new):
- Envelope: valid round-trip, wrong outer opcode, too-short body.
- Dispatcher: handler invoked, unhandled count, exception isolation,
  unregister + rollover to unhandled.
- Event parsers: ChannelBroadcast, Tell, UpdateHealth, WeenieError,
  Transient, MagicUpdateSpell.

Total: 521 tests pass (up from 508).

With this dispatcher in place, Phase F.2 (items + appraise), F.3 (combat
+ damage), F.4 (spell cast state machine), chat UI, allegiance, quest
tracker — all of which depend on GameEvent handling — are unblocked.

Ref: r08 §4 (GameEvent sub-opcode table), §2 (envelope wire shape).
Ref: ACE GameEventMessage.cs / GameEventType.cs.

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

src/AcDream.Core.Net/WorldSession.cs

@@ -115,6 +115,14 @@ public sealed class WorldSession : IDisposable
     /// <summary>Raised every time the state machine transitions.</summary>
     public event Action<State>? StateChanged;
 
+    /// <summary>
+    /// Phase F.1: inbound 0xF7B0 GameEvent dispatcher. Each sub-opcode
+    /// handler is registered here (by GameWindow / UI layer / chat
+    /// system) and routed on each incoming GameEvent. Unhandled
+    /// sub-opcodes are counted for diagnostic overlays.
+    /// </summary>
+    public GameEventDispatcher GameEvents { get; } = new();
+
     public State CurrentState { get; private set; } = State.Disconnected;
 
     /// <summary>Movement sequence counters for outbound MoveToState/AutonomousPosition.</summary>
@@ -479,6 +487,15 @@ public sealed class WorldSession : IDisposable
                         posUpdate.Value.Velocity));
                 }
             }
+            else if (op == GameEventEnvelope.Opcode)
+            {
+                // Phase F.1: 0xF7B0 is the GameEvent envelope. Parse the
+                // header (guid + sequence + eventType) and dispatch to the
+                // registered handler for that sub-opcode. Unregistered
+                // types get counted for diagnostic overlays.
+                var env = GameEventEnvelope.TryParse(body);
+                if (env is not null) GameEvents.Dispatch(env.Value);
+            }
             else if (op == 0xF751u)  // PlayerTeleport — server is moving us through a portal
             {
                 // Phase B.3: holtburger opcodes.rs confirms 0xF751 is the