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

@@ -0,0 +1,13 @@
+package com.mythlane.chainlightning.chain;
+
+import java.util.Optional;
+
+/**
+ * Ray-cast retournant la première entité touchée le long du rayon, ou empty.
+ * SAM permettant aux tests de fournir un résultat fixe et à Phase 3 de brancher
+ * l'API Hytale réelle.
+ */
+@FunctionalInterface
+public interface RayCaster {
+    Optional<ChainEntity> firstHit(Vec3 origin, Vec3 direction, double maxBlocks);
+}