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/main/java/com/mythlane/chainlightning/chain/VolumeCurve.java

@@ -0,0 +1,40 @@
+package com.mythlane.chainlightning.chain;
+
+/**
+ * Pure-Java volume curve for the chain-lightning sound emission.
+ * <p>
+ * Each successive hop in the chain emits its sound at a progressively lower
+ * volume, per VFX-02. The curve is hard-coded to the spec values
+ * {@code [1.0, 0.8, 0.6, 0.5, 0.4]} indexed by hop number.
+ * <p>
+ * This class has ZERO Hytale imports — Phase 2 sealed-frontier rule preserved.
+ */
+public final class VolumeCurve {
+
+    /** Spec curve from VFX-02. Order and values are exact. */
+    private static final float[] CURVE = {1.0f, 0.8f, 0.6f, 0.5f, 0.4f};
+
+    private VolumeCurve() {
+        // utility class — no instances
+    }
+
+    /**
+     * Volume for a given hop index in the chain.
+     * <ul>
+     *   <li>Indices 0..4 return the spec values {@code 1.0, 0.8, 0.6, 0.5, 0.4}.</li>
+     *   <li>Indices &ge; 5 clamp to the last value {@code 0.4f}.</li>
+     *   <li>Negative indices throw {@link IllegalArgumentException}.</li>
+     * </ul>
+     *
+     * @param hopIndex zero-based hop index
+     * @return volume scalar in the range (0, 1]
+     * @throws IllegalArgumentException if {@code hopIndex < 0}
+     */
+    public static float volumeFor(int hopIndex) {
+        if (hopIndex < 0) {
+            throw new IllegalArgumentException("hopIndex must be >= 0, got " + hopIndex);
+        }
+        int clamped = Math.min(hopIndex, CURVE.length - 1);
+        return CURVE[clamped];
+    }
+}