feat(04-01): add Vec3.lerp and ParticleTrail sampler with JUnit suite

- Vec3.lerp(other, t) for linear interpolation between two points
- ParticleTrail.sample(from, to, density) emits density-based interpolated
  points strictly between endpoints (endpoints excluded by construction)
- TRAIL_DENSITY = 4.0 particles per block
- 6 ParticleTrail tests + 3 Vec3.lerp tests, all green
- Zero Hytale imports — chain/ frontier preserved

src/test/java/com/mythlane/chainlightning/chain/Vec3Test.java

@@ -41,4 +41,34 @@ final class Vec3Test {
         assertEquals(12.0, a.distanceSquared(b), 1e-9);
         assertTrue(a.distance(b) > 3.4 && a.distance(b) < 3.5);
     }
+
+    @Test
+    void lerpAtZeroReturnsFrom() {
+        Vec3 from = Vec3.ZERO;
+        Vec3 to = new Vec3(10, 10, 10);
+        Vec3 result = from.lerp(to, 0.0);
+        assertEquals(from.x(), result.x(), 1e-9);
+        assertEquals(from.y(), result.y(), 1e-9);
+        assertEquals(from.z(), result.z(), 1e-9);
+    }
+
+    @Test
+    void lerpAtOneReturnsTo() {
+        Vec3 from = Vec3.ZERO;
+        Vec3 to = new Vec3(10, 10, 10);
+        Vec3 result = from.lerp(to, 1.0);
+        assertEquals(to.x(), result.x(), 1e-9);
+        assertEquals(to.y(), result.y(), 1e-9);
+        assertEquals(to.z(), result.z(), 1e-9);
+    }
+
+    @Test
+    void lerpAtHalfReturnsMidpoint() {
+        Vec3 from = new Vec3(0, 0, 0);
+        Vec3 to = new Vec3(10, 0, 0);
+        Vec3 result = from.lerp(to, 0.5);
+        assertEquals(5.0, result.x(), 1e-9);
+        assertEquals(0.0, result.y(), 1e-9);
+        assertEquals(0.0, result.z(), 1e-9);
+    }
 }