feat(03-06): add particle-system dump for discovery (Task 1)

- New DumpParticlesCommand utility: enumerates loaded ParticleSystem
  asset-ids via ParticleSystem.getAssetMap().getAssetMap().keySet().
- Wired in GravityFlipPlugin.start() behind GRAVITYFLIP_DUMP_PARTICLES
  env var (or -Dgravityflip.dumpParticles sysprop). No-op when unset.
- Boot-time fallback approach (plan-allowed) -- proper CommandBase
  registration deferred to avoid i18n translation-key plumbing for a
  one-shot discovery dump.
- See .planning/phases/03-gravity-physics/03-06-DUMP-NOTES.md for
  trigger instructions and the user checkpoint before Task 3.

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

@@ -5,6 +5,7 @@ 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.command.DumpParticlesCommand;
 import com.mythlane.gravityflip.config.GravityFlipConfig;
 import com.mythlane.gravityflip.physics.FallDamageGuard;
 import com.mythlane.gravityflip.physics.FallDamageSuppressorSystem;
@@ -103,6 +104,22 @@ public class GravityFlipPlugin extends JavaPlugin {
         getLogger().at(Level.INFO).log(
                 "Gravity Flip enabled — %d region(s) loaded, detector @100ms, gravity inversion active",
                 cfg.getRegions().size());
+
+        // Plan 03-06 Task 1 — one-shot ParticleSystem asset-id dump for default
+        // discovery. Enabled via env var GRAVITYFLIP_DUMP_PARTICLES=1 (or sysprop
+        // -Dgravityflip.dumpParticles=true). No-op otherwise. See
+        // DumpParticlesCommand javadoc for the chosen API rationale.
+        if (DumpParticlesCommand.isEnabled()) {
+            try {
+                int n = DumpParticlesCommand.dump(line ->
+                        getLogger().at(Level.INFO).log("%s", line));
+                getLogger().at(Level.INFO).log(
+                        "ParticleSystem dump complete: %d asset-ids logged", n);
+            } catch (Throwable th) {
+                getLogger().at(Level.WARNING).withCause(th)
+                        .log("ParticleSystem dump failed");
+            }
+        }
     }
 
     @Override