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/ParticleTrailTest.java

@@ -0,0 +1,82 @@
+package com.mythlane.chainlightning.chain;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+final class ParticleTrailTest {
+
+    private static final double EPS = 1e-9;
+
+    @Test
+    void zeroDistance() {
+        Vec3 p = new Vec3(2, 3, 4);
+        List<Vec3> out = ParticleTrail.sample(p, p, 4.0);
+        assertTrue(out.isEmpty());
+    }
+
+    @Test
+    void subBlockDistance() {
+        // distance 0.5 along X with density 4 → ceil(2.0) = 2
+        Vec3 from = new Vec3(0, 0, 0);
+        Vec3 to = new Vec3(0.5, 0, 0);
+        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
+        assertEquals(2, out.size());
+        // Both points strictly between endpoints
+        for (Vec3 v : out) {
+            assertTrue(from.distanceSquared(v) > EPS, "sample equals from");
+            assertTrue(to.distanceSquared(v) > EPS, "sample equals to");
+        }
+    }
+
+    @Test
+    void integerDistanceCount() {
+        Vec3 from = new Vec3(0, 0, 0);
+        Vec3 to = new Vec3(5, 0, 0);
+        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
+        assertEquals(20, out.size());
+    }
+
+    @Test
+    void fractionalDistanceCount() {
+        Vec3 from = new Vec3(0, 0, 0);
+        Vec3 to = new Vec3(3.7, 0, 0);
+        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
+        // ceil(3.7 * 4) = ceil(14.8) = 15
+        assertEquals(15, out.size());
+    }
+
+    @Test
+    void endpointsExcluded() {
+        Vec3 from = new Vec3(1, 2, 3);
+        Vec3 to = new Vec3(7, 8, 9);
+        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
+        assertTrue(out.size() > 0);
+        for (Vec3 v : out) {
+            assertTrue(from.distanceSquared(v) > EPS, "sample equals from endpoint");
+            assertTrue(to.distanceSquared(v) > EPS, "sample equals to endpoint");
+        }
+    }
+
+    @Test
+    void uniformSpacing() {
+        Vec3 from = new Vec3(0, 0, 0);
+        Vec3 to = new Vec3(5, 0, 0);
+        List<Vec3> out = ParticleTrail.sample(from, to, 4.0);
+        assertEquals(20, out.size());
+        double total = from.distance(to);
+        double expectedSpacing = total / (out.size() + 1);
+
+        // from -> first
+        assertEquals(expectedSpacing, from.distance(out.get(0)), EPS);
+        // last -> to
+        assertEquals(expectedSpacing, out.get(out.size() - 1).distance(to), EPS);
+        // consecutive
+        for (int i = 1; i < out.size(); i++) {
+            assertEquals(expectedSpacing, out.get(i - 1).distance(out.get(i)), EPS);
+        }
+    }
+}