feat(03-04): ajoute FallDamageGuard tracker thread-safe (Task 2)

- ConcurrentHashMap<UUID, region> current + exit + regionAtExit
- shouldSuppressFallDamage: in-region FallDamage=false OU grace window
- Pas de static mutable, injecté via constructeur
- 7 tests pure-data couvrant entry/in-region/exit/grace/re-entry/override

src/test/java/com/mythlane/gravityflip/physics/FallDamageGuardTest.java

@@ -0,0 +1,97 @@
+package com.mythlane.gravityflip.physics;
+
+import com.hypixel.hytale.math.shape.Box;
+import com.hypixel.hytale.math.vector.Vector3d;
+import com.mythlane.gravityflip.region.GravityFlipRegion;
+import org.junit.jupiter.api.Test;
+
+import java.util.UUID;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * Pure-data tests for {@link FallDamageGuard} — no Hytale runtime dependency.
+ * Covers entry / in-region / exit / grace-window / re-entry / FallDamage=true override.
+ */
+class FallDamageGuardTest {
+
+    @Test
+    void notInRegionReturnsFalse() {
+        FallDamageGuard guard = new FallDamageGuard();
+        assertFalse(guard.shouldSuppressFallDamage(UUID.randomUUID(), 1000L));
+    }
+
+    @Test
+    void inRegionFallDamageTrueReturnsFalse() {
+        FallDamageGuard guard = new FallDamageGuard();
+        UUID uuid = UUID.randomUUID();
+        GravityFlipRegion region = region(true, 2500);
+        guard.markInRegion(uuid, region);
+        assertFalse(guard.shouldSuppressFallDamage(uuid, 1000L));
+    }
+
+    @Test
+    void inRegionFallDamageFalseReturnsTrue() {
+        FallDamageGuard guard = new FallDamageGuard();
+        UUID uuid = UUID.randomUUID();
+        GravityFlipRegion region = region(false, 2500);
+        guard.markInRegion(uuid, region);
+        assertTrue(guard.shouldSuppressFallDamage(uuid, 1000L));
+    }
+
+    @Test
+    void afterExitWithinGraceReturnsTrue() {
+        FallDamageGuard guard = new FallDamageGuard();
+        UUID uuid = UUID.randomUUID();
+        GravityFlipRegion region = region(false, 2500);
+        guard.markInRegion(uuid, region);
+        long t0 = 1000L;
+        guard.markExit(uuid, region, t0);
+        assertTrue(guard.shouldSuppressFallDamage(uuid, t0 + 2499));
+    }
+
+    @Test
+    void afterExitBeyondGraceReturnsFalse() {
+        FallDamageGuard guard = new FallDamageGuard();
+        UUID uuid = UUID.randomUUID();
+        GravityFlipRegion region = region(false, 2500);
+        guard.markInRegion(uuid, region);
+        long t0 = 1000L;
+        guard.markExit(uuid, region, t0);
+        assertFalse(guard.shouldSuppressFallDamage(uuid, t0 + 2501));
+    }
+
+    @Test
+    void reEntryAfterExitResetsTracking() {
+        FallDamageGuard guard = new FallDamageGuard();
+        UUID uuid = UUID.randomUUID();
+        GravityFlipRegion region = region(false, 2500);
+        guard.markInRegion(uuid, region);
+        guard.markExit(uuid, region, 1000L);
+        guard.markInRegion(uuid, region);  // re-enter
+        // In-region again with FallDamage=false → immediate suppression, grace reset.
+        assertTrue(guard.shouldSuppressFallDamage(uuid, 1500L));
+    }
+
+    @Test
+    void fallDamageTrueRegionNeverSuppressesEvenDuringGrace() {
+        FallDamageGuard guard = new FallDamageGuard();
+        UUID uuid = UUID.randomUUID();
+        GravityFlipRegion suppressed = region(false, 2500);
+        GravityFlipRegion allowed = region(true, 2500);
+        guard.markInRegion(uuid, suppressed);
+        guard.markExit(uuid, suppressed, 1000L);
+        // New region has FallDamage=true → override immediately.
+        guard.markInRegion(uuid, allowed);
+        assertFalse(guard.shouldSuppressFallDamage(uuid, 1500L));
+    }
+
+    private static GravityFlipRegion region(boolean fallDamage, int graceMs) {
+        GravityFlipRegion r = new GravityFlipRegion(
+                "t", new Box(new Vector3d(0, 0, 0), new Vector3d(1, 1, 1)), true);
+        r.setFallDamage(fallDamage);
+        r.setGracePeriodMs(graceMs);
+        return r;
+    }
+}