refactor(03-06): remove debug logs + efficiency pass

Log cleanup:
- Drop redundant INFO "Gravity Flip enabled" in setup() (substantive version at
  start() remains as the single startup line).
- Drop INFO "debug enter/exit notifier ENABLED" (the notifier itself emits per
  region transition; extra startup noise not needed).
- Remove dead back-compat GravityApplier ctors that accepted an infoHandler
  Consumer (no callers post-03-06; debug logs already stripped).
- Clean stale javadoc referencing removed [DBG npc.woken]/[DBG npc.ctrlNull]
  one-shot logs.
- Also ship the pre-staged cleanup work: RegionEnterNotifier gated behind
  GRAVITYFLIP_DEBUG_NOTIFY env var, GravityApplier infoHandler field removed,
  tick logs stripped.

Efficiency:
- RegionVisualizer.resolveParticleId now memoises the requested->resolved
  mapping in a ConcurrentHashMap, eliminating the ParticleSystem.getAssetMap()
  lookup on every tick emission per region. Warn-once semantics preserved via
  warnedInvalidIds. Fail-open path (AssetMap unavailable in tests) intentionally
  does not populate the cache.
- Document in GravityApplier javadoc why Pass 1 + Pass 2 cannot be fused:
  restore requires the complete currentlyInRegion set before diffing against
  previouslyInverted.

Considered but not applied:
- ParticleEdgeEmitter.edgePoints caching per (box, density): throttled to
  >=100ms refresh and typical <20 regions => alloc pressure negligible;
  premature without measurement.
- Reusing RegionRegistry snapshot in RegionEnterNotifier: notifier is
  opt-in/off-by-default, so its independent ECS scan has zero prod cost.

Tests: ./gradlew clean build green, no test changes required.

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

@@ -7,6 +7,7 @@ import com.hypixel.hytale.server.core.universe.world.World;
 import com.hypixel.hytale.server.core.util.Config;
 import com.mythlane.gravityflip.command.DumpParticlesCommand;
 import com.mythlane.gravityflip.config.GravityFlipConfig;
+import com.mythlane.gravityflip.debug.RegionEnterNotifier;
 import com.mythlane.gravityflip.physics.FallDamageGuard;
 import com.mythlane.gravityflip.physics.FallDamageSuppressorSystem;
 import com.mythlane.gravityflip.physics.GravityApplier;
@@ -67,8 +68,6 @@ public class GravityFlipPlugin extends JavaPlugin {
         getEntityStoreRegistry().registerSystem(new FallDamageSuppressorSystem(
                 fallDamageGuard,
                 th -> getLogger().at(Level.WARNING).withCause(th).log("fallDamageSuppressor handle failed")));
-
-        getLogger().at(Level.INFO).log("Gravity Flip enabled");
     }
 
     @Override
@@ -78,13 +77,21 @@ public class GravityFlipPlugin extends JavaPlugin {
         this.registry = new RegionRegistry(cfg, configHolder);
         this.gravityApplier = new GravityApplier(
                 th -> getLogger().at(Level.WARNING).withCause(th).log("gravityApply failed"),
-                msg -> getLogger().at(Level.INFO).log("%s", msg),
                 fallDamageGuard);
         this.regionVisualizer = new RegionVisualizer(
                 th -> getLogger().at(Level.WARNING).withCause(th).log("regionVisualize failed"));
         this.tickLoop = new RegionTickLoop(registry, gravityApplier, regionVisualizer, th ->
                 getLogger().at(Level.WARNING).withCause(th).log("detectTick failed"));
 
+        // Debug enter/exit notifier — opt-in via env var GRAVITYFLIP_DEBUG_NOTIFY (or sysprop
+        // gravityflip.debugNotify). Emits chat + INFO log on every region transition. Off by
+        // default to keep prod chat/logs clean.
+        if (isDebugNotifyEnabled()) {
+            this.tickLoop.setEnterNotifier(new RegionEnterNotifier(registry,
+                    th -> getLogger().at(Level.WARNING).withCause(th).log("regionEnterNotify failed"),
+                    msg -> getLogger().at(Level.INFO).log("%s", msg)));
+        }
+
         // Lazy world resolution — see setup() comment.
         this.tickLoop.startWithDelay(2_000L, () -> {
             Universe u = Universe.get();
@@ -122,6 +129,21 @@ public class GravityFlipPlugin extends JavaPlugin {
         }
     }
 
+    /**
+     * Env var {@code GRAVITYFLIP_DEBUG_NOTIFY=1} or sysprop {@code -Dgravityflip.debugNotify=true}
+     * enables the chat + log enter/exit notifier. Off by default — debug tool only.
+     */
+    private static boolean isDebugNotifyEnabled() {
+        return truthy(System.getenv("GRAVITYFLIP_DEBUG_NOTIFY"))
+                || truthy(System.getProperty("gravityflip.debugNotify"));
+    }
+
+    private static boolean truthy(String v) {
+        if (v == null) return false;
+        String s = v.trim().toLowerCase();
+        return s.equals("1") || s.equals("true") || s.equals("yes") || s.equals("on");
+    }
+
     @Override
     protected void shutdown() {
         // Plan 03-05 : clear des debug shapes cote clients AVANT tickLoop.stop().