test(vfx #C.1.5a): real-emitter verification in OnRemove test + unused using

Code-review follow-up to 003c502:

1. Test 3 (OnRemove_StopsScriptsAndEmitters) now wires the runner into
   the real ParticleHookSink instead of a RecordingSink, registers a
   persistent EmitterDesc, lets the CreateParticleHook actually spawn an
   emitter, then asserts the sink killed it after OnRemove. Previously
   the test only verified runner-side state — sink.StopAllForEntity was
   never observably exercised, so a regression dropping that call would
   have passed silently.

2. Removed unused `using System.Numerics` from EntityScriptActivator.cs.

No production code changes. Tests 1 and 2 unchanged.

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

src/AcDream.App/Rendering/Vfx/EntityScriptActivator.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Numerics;
 using AcDream.Core.Vfx;
 using AcDream.Core.World;