feat(04-01): add VolumeCurve with hardcoded VFX-02 spec curve

- VolumeCurve.volumeFor(int) returns spec values [1.0, 0.8, 0.6, 0.5, 0.4]
- High indices clamp to 0.4f (last value)
- Negative indices throw IllegalArgumentException
- 3 JUnit cases (specValues, indexClamp, negativeIndexThrows), all green
- Zero Hytale imports — chain/ frontier preserved

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

@@ -0,0 +1,31 @@
+package com.mythlane.chainlightning.chain;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+final class VolumeCurveTest {
+
+    @Test
+    void specValues() {
+        assertEquals(1.0f, VolumeCurve.volumeFor(0), 1e-6);
+        assertEquals(0.8f, VolumeCurve.volumeFor(1), 1e-6);
+        assertEquals(0.6f, VolumeCurve.volumeFor(2), 1e-6);
+        assertEquals(0.5f, VolumeCurve.volumeFor(3), 1e-6);
+        assertEquals(0.4f, VolumeCurve.volumeFor(4), 1e-6);
+    }
+
+    @Test
+    void indexClamp() {
+        assertEquals(0.4f, VolumeCurve.volumeFor(5), 1e-6);
+        assertEquals(0.4f, VolumeCurve.volumeFor(99), 1e-6);
+        assertEquals(0.4f, VolumeCurve.volumeFor(Integer.MAX_VALUE), 1e-6);
+    }
+
+    @Test
+    void negativeIndexThrows() {
+        assertThrows(IllegalArgumentException.class, () -> VolumeCurve.volumeFor(-1));
+        assertThrows(IllegalArgumentException.class, () -> VolumeCurve.volumeFor(Integer.MIN_VALUE));
+    }
+}