feat(phys L.2g slice 1): ShadowObjectRegistry.UpdatePhysicsState

New mutator that overwrites cached PhysicsState bits on every shadow copy
of the named entity. The existing CollisionExemption.ShouldSkip(...) check
(acclient_2013_pseudo_c.txt:276782) reads the same cached field, so a
post-spawn ETHEREAL flip is now honored on the next resolver tick without
any resolver-path change.

Retail anchor: CPhysicsObj::set_state at acclient_2013_pseudo_c.txt:283044.
Slice 1 scopes to the bare state-write — retail's cosmetic side-effect
handlers (0x800 lighting, 0x20 nodraw, 0x4000 hidden) don't fire for the
ETHEREAL bit and stay deferred.

Three TDD tests cover: ETHEREAL flip from 0->0x4; unregistered-entity
no-op; entity spanning multiple cells gets all copies updated.

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

src/AcDream.Core/Physics/ShadowObjectRegistry.cs

@@ -126,6 +126,47 @@ public sealed class ShadowObjectRegistry
                  t.State, t.Flags);
     }
 
+    /// <summary>
+    /// Update the cached <see cref="ShadowEntry.State"/> bits for an
+    /// already-registered entity. Called by the inbound
+    /// <c>SetState (0xF74B)</c> dispatcher when the server broadcasts a
+    /// post-spawn <c>PhysicsState</c> change — chiefly doors flipping
+    /// <c>ETHEREAL_PS = 0x4</c> on Use, so the
+    /// <see cref="CollisionExemption.ShouldSkip"/> short-circuit can honor
+    /// the new state on the next resolve.
+    ///
+    /// <para>
+    /// Retail equivalent: <c>CPhysicsObj::set_state</c> at
+    /// <c>docs/research/named-retail/acclient_2013_pseudo_c.txt:283044</c>
+    /// — direct write `this->state = arg2`. Retail also fires side-effect
+    /// handlers for the 0x800 (lighting), 0x20 (nodraw), 0x4000 (hidden)
+    /// changed bits; ETHEREAL (0x4) doesn't trigger any of them, so slice 1
+    /// scopes to the bare state-write.
+    /// </para>
+    ///
+    /// <para>
+    /// Implementation: <see cref="ShadowEntry"/> is a value-type record
+    /// copied into per-cell lists, so we rewrite the copy in each cell the
+    /// entity occupies. Unregistered entities are a no-op (callers don't
+    /// have to gate).
+    /// </para>
+    /// </summary>
+    public void UpdatePhysicsState(uint entityId, uint newState)
+    {
+        if (!_entityToCells.TryGetValue(entityId, out var cellIds))
+            return; // not registered — no-op
+
+        foreach (var cellId in cellIds)
+        {
+            if (!_cells.TryGetValue(cellId, out var list)) continue;
+            for (int i = 0; i < list.Count; i++)
+            {
+                if (list[i].EntityId == entityId)
+                    list[i] = list[i] with { State = newState };
+            }
+        }
+    }
+
     /// <summary>Remove an entity from all cells it was registered in.</summary>
     public void Deregister(uint entityId)
     {