feat(phys L.2g slice 1): WorldSession dispatches SetState (0xF74B) + hex probe

Three changes folded into one commit:

1. New public StateUpdated event on WorldSession + dispatcher branch
   for op == SetState.Opcode. Mirrors the VectorUpdated /
   MotionUpdated event pattern. GameWindow will subscribe in the next
   commit and feed the parsed (guid, newState) pair to
   ShadowObjectRegistry.UpdatePhysicsState.

2. One-shot probe-gated hex-dump (ACDREAM_PROBE_BUILDING) emits the
   first inbound SetState message's body bytes. Originally planned
   as a separate slice 1.5 confidence-check on holtburger's claimed
   12-byte payload vs ACE's GameMessageSetState.cs. Folded into the
   dispatcher to avoid re-touching the same branch. The new
   _setStateHexDumped guard keeps the log clean — auto-close every
   30s would otherwise produce noise.

3. Doc-comment polish on SetState.cs requested by Task 1's code
   review: remove false uncertainty about ACE's sequence-field width
   (ACE's UShortSequence.CurrentBytes provably writes 2 bytes via
   BitConverter), and align the 'total body size' phrasing with
   VectorUpdate.cs's convention. Folded here to avoid churning the
   file twice this slice.

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

src/AcDream.Core.Net/WorldSession.cs

@@ -129,6 +129,19 @@ public sealed class WorldSession : IDisposable
     /// </summary>
     public event Action<VectorUpdate.Parsed>? VectorUpdated;
 
+    /// <summary>
+    /// Fires when the server broadcasts a <c>SetState (0xF74B)</c> game
+    /// message — a previously-spawned entity's <c>PhysicsState</c>
+    /// bitmask changed post-CreateObject. Chiefly doors flipping
+    /// <c>ETHEREAL_PS = 0x4</c> on Use (see ACE
+    /// <c>WorldObjects/Door.cs:127</c>, <c>WorldObject.cs:640-660</c>).
+    /// Subscribers route the new state into
+    /// <see cref="ShadowObjectRegistry.UpdatePhysicsState"/> so the
+    /// existing collision-exemption short-circuit honors the flip on the
+    /// next resolver tick.
+    /// </summary>
+    public event Action<SetState.Parsed>? StateUpdated;
+
     /// <summary>
     /// Fires when the server sends a PlayerTeleport (0xF751) game message,
     /// signalling that the player is entering portal space. The uint payload
@@ -375,6 +388,10 @@ public sealed class WorldSession : IDisposable
     /// </summary>
     private bool _loginCompleteSent;
 
+    /// <summary>L.2g slice 1: one-shot guard so the [setstate-hex] probe
+    /// emits the first SetState's body bytes only, not 5–10/sec.</summary>
+    private bool _setStateHexDumped;
+
     /// <summary>
     /// Phase B.2: per-session game-action sequence counter. Monotonically
     /// incremented by <see cref="NextGameActionSequence"/> and embedded in
@@ -750,6 +767,28 @@ public sealed class WorldSession : IDisposable
                 if (parsed is not null)
                     VectorUpdated?.Invoke(parsed.Value);
             }
+            else if (op == SetState.Opcode)
+            {
+                // L.2g slice 1 (2026-05-12): server broadcasts SetState
+                // (0xF74B) when an entity's PhysicsState changes
+                // post-spawn — chiefly doors flipping ETHEREAL on Use.
+                // Holtburger validated wire format = 16 bytes (opcode +
+                // guid + state + 2×u16 sequence). One-shot probe-gated
+                // hex-dump (ACDREAM_PROBE_BUILDING) captures the wire
+                // bytes for confidence before declaring slice 1 done.
+                if (AcDream.Core.Physics.PhysicsDiagnostics.ProbeBuildingEnabled
+                    && !_setStateHexDumped)
+                {
+                    _setStateHexDumped = true;
+                    var hex = string.Join(" ", body.Take(Math.Min(body.Length, 32))
+                        .Select(b => b.ToString("X2")));
+                    Console.WriteLine($"[setstate-hex] body.len={body.Length} first-{Math.Min(body.Length, 32)}-bytes: {hex}");
+                }
+
+                var parsed = SetState.TryParse(body);
+                if (parsed is not null)
+                    StateUpdated?.Invoke(parsed.Value);
+            }
             else if (op == HearSpeech.LocalOpcode || op == HearSpeech.RangedOpcode)
             {
                 // Phase H.1: local/ranged chat. Standalone GameMessage