fix(vfx #56): ParticleHookSink applies CreateParticleHook.PartIndex transform

Adds per-entity part-transform side-table mirroring _rotationByEntity.
SpawnFromHook now transforms the hook offset through partTransforms[partIndex]
before rotating to world space. Backwards-compatible: entities without
registered part transforms fall through to identity (pre-C.1.5b behavior),
so the existing C.1.5a rotation-seed test stays green.

Adds SetEntityPartTransforms public method. Cleared on StopAllForEntity
alongside the rotation entry.

2 new xUnit tests:
- SpawnFromHook_AppliesPartTransform_WhenRegistered — part 1 lifted +Z=1,
  hook offset (1,0,0), PartIndex=1 → world (1,0,1).
- SpawnFromHook_FallsBackToIdentity_WhenPartIndexOutOfBounds — PartIndex=99
  on a 2-part array → offset applied without crash, pre-C.1.5b behavior.

Closes the renderer side of #56. EntityScriptActivator wiring (Task 3)
lands next.

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

src/AcDream.Core/Vfx/ParticleHookSink.cs

@@ -72,6 +72,14 @@ public sealed class ParticleHookSink : IAnimationHookSink
     private readonly ConcurrentDictionary<int, (uint EntityId, uint KeyId)> _trackingByHandle = new();
     private readonly ConcurrentDictionary<uint, ParticleRenderPass> _renderPassByEntity = new();
     private readonly ConcurrentDictionary<uint, Quaternion> _rotationByEntity = new();
+    // C.1.5b #56: per-entity static part transforms (PlacementFrames[Resting]
+    // baked into a Matrix4x4 per Setup part). When set, SpawnFromHook applies
+    // partTransforms[hook.PartIndex] to the hook offset BEFORE rotating to
+    // world space. Without this, every emitter in a multi-part Setup
+    // collapses to the entity root (the bug). Cleared by StopAllForEntity.
+    // For ANIMATED entities this map would need a per-tick refresh similar
+    // to UpdateEntityAnchor — deferred to a future phase.
+    private readonly ConcurrentDictionary<uint, IReadOnlyList<Matrix4x4>> _partTransformsByEntity = new();
     private int _anonymousEmitterSerial;
 
     public ParticleHookSink(ParticleSystem system)
@@ -131,6 +139,19 @@ public sealed class ParticleHookSink : IAnimationHookSink
     public void SetEntityRotation(uint entityId, Quaternion rotation)
         => _rotationByEntity[entityId] = rotation;
 
+    /// <summary>
+    /// Register per-part static transforms for an entity. The caller
+    /// (typically <c>EntityScriptActivator</c>) precomputes one
+    /// <see cref="Matrix4x4"/> per Setup part using
+    /// <c>SetupPartTransforms.Compute</c> and pushes them here at spawn
+    /// time. <see cref="SpawnFromHook"/> applies
+    /// <c>partTransforms[hook.PartIndex]</c> to the hook offset BEFORE
+    /// transforming to world space. Cleared on
+    /// <see cref="StopAllForEntity"/>.
+    /// </summary>
+    public void SetEntityPartTransforms(uint entityId, IReadOnlyList<Matrix4x4> partTransforms)
+        => _partTransformsByEntity[entityId] = partTransforms;
+
     public void ClearEntityRenderPass(uint entityId)
         => _renderPassByEntity.TryRemove(entityId, out _);
 
@@ -171,6 +192,7 @@ public sealed class ParticleHookSink : IAnimationHookSink
 
         ClearEntityRenderPass(entityId);
         _rotationByEntity.TryRemove(entityId, out _);
+        _partTransformsByEntity.TryRemove(entityId, out _);
     }
 
     private void SpawnFromHook(
@@ -181,13 +203,22 @@ public sealed class ParticleHookSink : IAnimationHookSink
         int partIndex,
         uint logicalId)
     {
-        // Spawn position: entity pose + hook offset. PartIndex will be
-        // used when the renderer passes per-part transforms through; for
-        // now, fold it into the root pos.
+        // Spawn position: entity pose + hook offset, with the hook
+        // offset first passed through the per-part transform when
+        // available (C.1.5b #56 fix). Without the per-part transform,
+        // every emitter in a multi-emitter PES script collapses to the
+        // entity root — visible symptom: ground-buried portal swirls.
         var rotation = _rotationByEntity.TryGetValue(entityId, out var rot)
             ? rot
             : Quaternion.Identity;
-        var anchor = worldPos + Vector3.Transform(offset, rotation);
+        Vector3 partLocal = offset;
+        if (_partTransformsByEntity.TryGetValue(entityId, out var partTransforms)
+            && partIndex >= 0
+            && partIndex < partTransforms.Count)
+        {
+            partLocal = Vector3.Transform(offset, partTransforms[partIndex]);
+        }
+        var anchor = worldPos + Vector3.Transform(partLocal, rotation);
         var renderPass = _renderPassByEntity.TryGetValue(entityId, out var pass)
             ? pass
             : ParticleRenderPass.Scene;