feat(02-02): add RegionTickLoop + plugin wiring (Task 3)

- RegionTickLoop: single-thread daemon scheduler @100ms, 2s initial delay,
  shutdown -> awaitTermination(5s) -> shutdownNow lifecycle.
  Three start overloads: (World), (Supplier<World>), (Runnable). Errors are
  routed to a Consumer<Throwable> so an exception in one tick never kills
  the scheduler.
- GravityFlipPlugin.start():
  * builds RegionRegistry from configHolder.get() + holder for save()
  * starts the tick loop with a Supplier<World> that lazily resolves
    Universe.get().getDefaultWorld() (deviation: PrepareUniverseEvent in the
    pinned API only carries WorldConfigProvider — no Universe/World access,
    so we use the lazy-supplier pattern from MythWorld instead)
  * registers the ScheduledFuture with TaskRegistry (raw cast, try/catch fallback)
- GravityFlipPlugin.shutdown(): tickLoop.stop() BEFORE super.shutdown()
- RegionTickLoopTest: 3 timing tests pass (>=8 ticks/sec, stop within 5s,
  exception resilience).
- gradle build green; fat jar contains both region/ and tick/ class dirs.

src/main/java/com/mythlane/gravityflip/GravityFlipPlugin.java

@@ -2,9 +2,14 @@ package com.mythlane.gravityflip;
 
 import com.hypixel.hytale.server.core.plugin.JavaPlugin;
 import com.hypixel.hytale.server.core.plugin.JavaPluginInit;
+import com.hypixel.hytale.server.core.universe.Universe;
+import com.hypixel.hytale.server.core.universe.world.World;
 import com.hypixel.hytale.server.core.util.Config;
 import com.mythlane.gravityflip.config.GravityFlipConfig;
+import com.mythlane.gravityflip.region.RegionRegistry;
+import com.mythlane.gravityflip.tick.RegionTickLoop;
 
+import java.util.concurrent.ScheduledFuture;
 import java.util.logging.Level;
 
 /**
@@ -29,6 +34,9 @@ public class GravityFlipPlugin extends JavaPlugin {
     private final Config<GravityFlipConfig> configHolder =
             withConfig("regions", GravityFlipConfig.CODEC);
 
+    private RegionRegistry registry;
+    private RegionTickLoop tickLoop;
+
     public GravityFlipPlugin(JavaPluginInit init) {
         super(init);
     }
@@ -37,17 +45,60 @@ public class GravityFlipPlugin extends JavaPlugin {
     protected void setup() {
         // NOTE: do NOT call configHolder.get() here — it blocks until preLoad() completes.
         // Safe call sites are start() and any later lifecycle phase (incl. tick loop).
+        //
+        // World acquisition note (Phase 02-02): the plan called for a PrepareUniverseEvent
+        // listener that stashes a World reference. Empirically, PrepareUniverseEvent
+        // (com.hypixel.hytale.server.core.event.events.PrepareUniverseEvent) only carries
+        // a WorldConfigProvider — it does NOT expose a Universe or World. We therefore use
+        // a Supplier<World> that resolves Universe.get().getDefaultWorld() lazily on each
+        // tick (matching the MythWorld WorldBorderManager precedent). Until the universe
+        // is ready, the supplier returns null and the tick is a no-op.
         getLogger().at(Level.INFO).log("Gravity Flip enabled");
     }
 
+    @Override
+    protected void start() {
+        super.start();
+        GravityFlipConfig cfg = configHolder.get();
+        this.registry = new RegionRegistry(cfg, configHolder);
+        this.tickLoop = new RegionTickLoop(registry, th ->
+                getLogger().at(Level.WARNING).withCause(th).log("detectTick failed"));
+
+        // Lazy world resolution — see setup() comment.
+        this.tickLoop.startWithDelay(2_000L, () -> {
+            Universe u = Universe.get();
+            return u == null ? null : u.getDefaultWorld();
+        });
+
+        // TaskRegistry registration: registerTask only accepts ScheduledFuture<Void>; the
+        // scheduler returns ScheduledFuture<?>. Cast via raw types per Mythlane idiom; the
+        // try/catch falls back to manual shutdown() if registration fails (deterministic
+        // either way because shutdown() always invokes tickLoop.stop()).
+        try {
+            @SuppressWarnings({"unchecked", "rawtypes"})
+            ScheduledFuture<Void> vf = (ScheduledFuture) tickLoop.future();
+            getTaskRegistry().registerTask(vf);
+        } catch (Throwable ignored) { /* manual shutdown() fallback */ }
+
+        getLogger().at(Level.INFO).log(
+                "Gravity Flip enabled — %d region(s) loaded, detector @100ms",
+                cfg.getRegions().size());
+    }
+
     @Override
     protected void shutdown() {
+        // Stop the detector BEFORE super.shutdown() so no tick races plugin teardown.
+        if (tickLoop != null) tickLoop.stop();
         // No auto-save contract: any mutation made during the session must already
         // have been persisted via configHolder().save() by the command handler that
         // performed it. See configHolder() javadoc.
         getLogger().at(Level.INFO).log("Gravity Flip disabled");
+        super.shutdown();
     }
 
+    /** Exposed for Phase 3 (gravity physics) and Phase 4 (commands). */
+    public RegionRegistry regions() { return registry; }
+
     /**
      * Accessor for the region config holder. <strong>SAVE CONTRACT:</strong> any
      * caller that mutates {@code configHolder().get().getRegions()} MUST call