feat(phase-2): pure-Java chain resolution algorithm

Adds com.mythlane.chainlightning.chain package with 7 types (Vec3,
ChainEntity, EntitySource, RayCaster, ChainParameters, ChainHit,
ChainResolver). Algorithm: ray-cast primary target then BFS hops with
distanceSquared closest-neighbor selection, deterministic lexicographic
tie-breaker on entity id, max 5 targets, 8-block radius, damage curve
[8,6,4,3,2]. Strict no-Hytale-imports boundary — runtime adapters land
in Phase 3.

JUnit 5 suite: 25 tests green (Vec3 5 + ChainParameters 10 +
ChainResolver 10). All 10 mandatory cases covered (no primary,
primary-only, full chain, overflow, out-of-radius, no-double-hit,
closest, tie-breaker determinism, dead entity, custom maxTargets).

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

@@ -0,0 +1,16 @@
+package com.mythlane.chainlightning.chain;
+
+/**
+ * Helper test record implémentant ChainEntity. Permet construction concise dans
+ * les tests : entity("e1", 0, 0, 0) pour un vivant, dead("e2", 5, 0, 0) pour un mort.
+ */
+record TestChainEntity(String id, Vec3 position, boolean isAlive) implements ChainEntity {
+
+    static TestChainEntity entity(String id, double x, double y, double z) {
+        return new TestChainEntity(id, new Vec3(x, y, z), true);
+    }
+
+    static TestChainEntity dead(String id, double x, double y, double z) {
+        return new TestChainEntity(id, new Vec3(x, y, z), false);
+    }
+}