feat(04-00): minimal chain_spark particle assets for codec spike

- Author chain_spark.particlesystem (Spawners[1] -> SpawnerId=chain_spark)
- Author chain_spark.particlespawner (required co-asset, Sphere/BlendAdd, embedded Particle with Animation[0,100])
- Bundle 4x4 placeholder chain_spark.png in Particles/ to satisfy CommonAssetValidator.TEXTURE_PARTICLES
- ShadowJar packages all three at exact case-sensitive path Server/Particles/

Deviation from plan: research assumed only one .particlesystem file needed. Decompiled codec
shows ParticleSpawner is a SEPARATE asset (.particlespawner) referenced by SpawnerId, plus
Particle.Texture is validated against CommonAssetRegistry (must exist as Particles/<name>.png).
Both required for the spike to validate end-to-end. Documented in SPIKE-FINDINGS.

src/main/resources/Server/Particles/chain_spark.particlespawner

@@ -0,0 +1,32 @@
+{
+  "Shape": "Sphere",
+  "ParticleRotationInfluence": "Billboard",
+  "RenderMode": "BlendAdd",
+  "TotalParticles": { "Min": 1, "Max": 1 },
+  "LifeSpan": 0.5,
+  "MaxConcurrentParticles": 16,
+  "ParticleLifeSpan": { "Min": 0.3, "Max": 0.5 },
+  "SpawnRate": { "Min": 50.0, "Max": 50.0 },
+  "Particle": {
+    "Texture": "Particles/chain_spark.png",
+    "FrameSize": { "Width": 4, "Height": 4 },
+    "SoftParticles": "Enable",
+    "SoftParticlesFadeFactor": 1.0,
+    "UVOption": "None",
+    "ScaleRatioConstraint": "OneToOne",
+    "Animation": {
+      "0": {
+        "FrameIndex": { "Min": 0, "Max": 0 },
+        "Scale": { "X": { "Min": 0.3, "Max": 0.3 }, "Y": { "Min": 0.3, "Max": 0.3 } },
+        "Color": "#B0DCFF",
+        "Opacity": 1.0
+      },
+      "100": {
+        "FrameIndex": { "Min": 0, "Max": 0 },
+        "Scale": { "X": { "Min": 0.0, "Max": 0.0 }, "Y": { "Min": 0.0, "Max": 0.0 } },
+        "Color": "#B0DCFF",
+        "Opacity": 0.0
+      }
+    }
+  }
+}

src/main/resources/Server/Particles/chain_spark.particlesystem

@@ -0,0 +1,10 @@
+{
+  "LifeSpan": 0.5,
+  "CullDistance": 64.0,
+  "BoundingRadius": 1.0,
+  "Spawners": [
+    {
+      "SpawnerId": "chain_spark"
+    }
+  ]
+}