feat(vfx #C.1.5a): add EntityScriptActivator (no wiring yet)

New ~50-line orchestrator that fires Setup.DefaultScript through the
already-shipped PhysicsScriptRunner on entity spawn and stops scripts +
live emitters on despawn. Resolver delegate avoids DatCollection coupling
so the class is fully unit-testable with stubs.

Three xUnit tests cover the three branches: fire-with-script,
no-op-without-script, stop-on-remove. No wiring into the live spawn path
yet -- that lands in the next commit.

Spec: docs/superpowers/specs/2026-05-12-phase-c1.5a-portals-design.md

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

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

@@ -0,0 +1,86 @@
+using System;
+using System.Numerics;
+using AcDream.Core.Vfx;
+using AcDream.Core.World;
+
+namespace AcDream.App.Rendering.Vfx;
+
+/// <summary>
+/// Fires <c>Setup.DefaultScript</c> through <see cref="PhysicsScriptRunner"/>
+/// when a server-spawned <see cref="WorldEntity"/> enters the world, so static
+/// objects (portals, chimneys, fireplaces, building details) emit their
+/// retail-faithful persistent particle effects automatically. Stops the
+/// scripts and live emitters when the entity despawns.
+///
+/// <para>
+/// Wires alongside <c>EntitySpawnAdapter</c> in <c>GpuWorldState</c>: the
+/// adapter handles meshes + animation state, the activator handles scripts +
+/// particles. Both are render-thread-only.
+/// </para>
+///
+/// <para>
+/// Retail oracle: <c>play_script_internal(setup.DefaultScript)</c> is what
+/// retail's <c>CPhysicsObj</c> invokes at object load (see Phase C.1 plan §C.1
+/// and <c>memory/project_sky_pes_port.md</c>). C.1 already shipped the runner;
+/// this class adds the missing fire-on-spawn call site.
+/// </para>
+/// </summary>
+public sealed class EntityScriptActivator
+{
+    private readonly PhysicsScriptRunner _scriptRunner;
+    private readonly ParticleHookSink _particleSink;
+    private readonly Func<WorldEntity, uint> _defaultScriptResolver;
+
+    /// <param name="scriptRunner">Already-shipped runner from C.1. Owns the
+    /// (scriptId, entityId) instance table and schedules hooks at their
+    /// <c>StartTime</c> offsets.</param>
+    /// <param name="particleSink">Already-shipped hook sink from C.1. The
+    /// activator only calls its <see cref="ParticleHookSink.StopAllForEntity"/>
+    /// to drop any per-entity emitter handles on despawn.</param>
+    /// <param name="defaultScriptResolver">Returns
+    /// <c>entity.SourceGfxObjOrSetupId</c>'s <c>Setup.DefaultScript.DataId</c>,
+    /// or <c>0</c> on miss / dat throw / missing field. Production lambda hits
+    /// <see cref="DatReaderWriter.DatCollection"/>; tests pass a hand-rolled
+    /// stub.</param>
+    public EntityScriptActivator(
+        PhysicsScriptRunner scriptRunner,
+        ParticleHookSink particleSink,
+        Func<WorldEntity, uint> defaultScriptResolver)
+    {
+        ArgumentNullException.ThrowIfNull(scriptRunner);
+        ArgumentNullException.ThrowIfNull(particleSink);
+        ArgumentNullException.ThrowIfNull(defaultScriptResolver);
+        _scriptRunner = scriptRunner;
+        _particleSink = particleSink;
+        _defaultScriptResolver = defaultScriptResolver;
+    }
+
+    /// <summary>
+    /// Resolve the entity's <c>Setup.DefaultScript</c> and fire it through
+    /// the script runner. No-op if the entity has no DefaultScript
+    /// (resolver returns 0) or if the entity has no server guid
+    /// (atlas-tier entities are out of scope for this activator).
+    /// </summary>
+    public void OnCreate(WorldEntity entity)
+    {
+        ArgumentNullException.ThrowIfNull(entity);
+        if (entity.ServerGuid == 0) return;
+
+        uint scriptId = _defaultScriptResolver(entity);
+        if (scriptId == 0) return;
+
+        _scriptRunner.Play(scriptId, entity.ServerGuid, entity.Position);
+    }
+
+    /// <summary>
+    /// Stop every script instance the runner is tracking for this entity, and
+    /// kill every live emitter the sink has attributed to it. Idempotent for
+    /// unknown guids (both calls no-op).
+    /// </summary>
+    public void OnRemove(uint serverGuid)
+    {
+        if (serverGuid == 0) return;
+        _scriptRunner.StopAllForEntity(serverGuid);
+        _particleSink.StopAllForEntity(serverGuid, fadeOut: false);
+    }
+}