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>

tests/AcDream.Core.Tests/Vfx/ParticleHookSinkTests.cs

@@ -92,4 +92,84 @@ public sealed class ParticleHookSinkTests
         sys.Tick(0.01f);
         Assert.Equal(0, sys.ActiveEmitterCount);
     }
+
+    [Fact]
+    public void SpawnFromHook_AppliesPartTransform_WhenRegistered()
+    {
+        // C.1.5b #56: when SetEntityPartTransforms has been called for
+        // entityId, SpawnFromHook must transform the hook offset through
+        // the part-local matrix before applying entity rotation.
+        // Part 1 is lifted +Z=1; hook offset = (1, 0, 0), PartIndex=1.
+        // Expected world position: (1, 0, 1) with identity rotation.
+        var registry = new EmitterDescRegistry();
+        registry.Register(MakeDesc(0x32000030u, attachLocal: false));
+        var sys = new ParticleSystem(registry, new System.Random(42));
+        var sink = new ParticleHookSink(sys);
+
+        var partTransforms = new Matrix4x4[]
+        {
+            Matrix4x4.Identity,
+            Matrix4x4.CreateTranslation(0f, 0f, 1f),
+        };
+        sink.SetEntityRotation(0xCAFEu, Quaternion.Identity);
+        sink.SetEntityPartTransforms(0xCAFEu, partTransforms);
+
+        sink.OnHook(0xCAFEu, Vector3.Zero, new CreateParticleHook
+        {
+            EmitterInfoId = 0x32000030u,
+            EmitterId = 0,
+            PartIndex = 1,
+            Offset = new Frame
+            {
+                Origin = new Vector3(1f, 0f, 0f),
+                Orientation = Quaternion.Identity,
+            },
+        });
+        sys.Tick(0.001f);
+
+        var live = System.Linq.Enumerable.Single(sys.EnumerateLive());
+        var pos = live.Emitter.Particles[live.Index].Position;
+        Assert.InRange(pos.X, 0.99f, 1.01f);
+        Assert.InRange(pos.Y, -0.01f, 0.01f);
+        Assert.InRange(pos.Z, 0.99f, 1.01f);
+    }
+
+    [Fact]
+    public void SpawnFromHook_FallsBackToIdentity_WhenPartIndexOutOfBounds()
+    {
+        // Out-of-bounds PartIndex must NOT crash and must NOT apply a
+        // wrong matrix; falls back to no part transform (Identity), so
+        // the offset is applied in entity-local space as pre-C.1.5b.
+        var registry = new EmitterDescRegistry();
+        registry.Register(MakeDesc(0x32000031u, attachLocal: false));
+        var sys = new ParticleSystem(registry, new System.Random(42));
+        var sink = new ParticleHookSink(sys);
+
+        var partTransforms = new Matrix4x4[]
+        {
+            Matrix4x4.Identity,
+            Matrix4x4.CreateTranslation(0f, 0f, 1f),
+        };
+        sink.SetEntityRotation(0xCAFEu, Quaternion.Identity);
+        sink.SetEntityPartTransforms(0xCAFEu, partTransforms);
+
+        sink.OnHook(0xCAFEu, Vector3.Zero, new CreateParticleHook
+        {
+            EmitterInfoId = 0x32000031u,
+            EmitterId = 0,
+            PartIndex = 99,  // way past the 2-part array
+            Offset = new Frame
+            {
+                Origin = new Vector3(2f, 0f, 0f),
+                Orientation = Quaternion.Identity,
+            },
+        });
+        sys.Tick(0.001f);
+
+        var live = System.Linq.Enumerable.Single(sys.EnumerateLive());
+        var pos = live.Emitter.Particles[live.Index].Position;
+        Assert.InRange(pos.X, 1.99f, 2.01f);
+        Assert.InRange(pos.Y, -0.01f, 0.01f);
+        Assert.InRange(pos.Z, -0.01f, 0.01f);
+    }
 }