feat(02-01): wire GravityFlipConfig to regions.json via named withConfig

- GravityFlipConfig wraps List<GravityFlipRegion> via ArrayCodec under the
  KeyedCodec("Regions", ...) entry. Decoded list is a mutable ArrayList
  (NOT List.of(arr)) so Phase 4 commands can mutate at runtime.
- GravityFlipPlugin.configHolder uses the named overload
  withConfig("regions", GravityFlipConfig.CODEC) — produces
  <dataDirectory>/regions.json. The 1-arg overload would hardcode config.json.
- configHolder() javadoc documents the save contract (no auto-save on
  shutdown; mutators must call configHolder.save() explicitly) and the shared
  mutable reference caveat that 02-02's RegionRegistry will resolve.
- 4 round-trip tests cover: empty default, two-region order preservation,
  empty list, and list mutability (regression guard against List.of).

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

@@ -2,6 +2,8 @@ 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.util.Config;
+import com.mythlane.gravityflip.config.GravityFlipConfig;
 
 import java.util.logging.Level;
 
@@ -9,25 +11,53 @@ import java.util.logging.Level;
  * Entry point for the Gravity Flip plugin.
  *
  * <p>Extends {@link JavaPlugin} from the resolved Hytale Server API
- * ({@code com.hypixel.hytale:Server:2026.03.26-89796e57b}). The lifecycle
- * hooks in this API version are {@code setup()} and {@code shutdown()}
- * (NOT {@code onEnable()} / {@code onDisable()}, which belong to older
- * docs). See {@code .planning/phases/01-scaffold-load/JAVAPLUGIN_RESOLUTION.md}
- * for the empirical resolution.
+ * ({@code com.hypixel.hytale:Server:2026.03.26-89796e57b}). Lifecycle hooks
+ * for this version are {@code setup()} / {@code shutdown()} (not the legacy
+ * {@code onEnable()} / {@code onDisable()}). See
+ * {@code .planning/phases/01-scaffold-load/JAVAPLUGIN_RESOLUTION.md} for the
+ * empirical resolution.
  */
 public class GravityFlipPlugin extends JavaPlugin {
 
+    /**
+     * Persisted region store, materialised on disk as
+     * {@code <dataDirectory>/regions.json} (i.e. {@code Plugins/GravityFlip/regions.json}).
+     *
+     * <p>The named {@code withConfig(name, codec)} overload is REQUIRED — the
+     * 1-arg overload hardcodes the filename to {@code config.json}.
+     */
+    private final Config<GravityFlipConfig> configHolder =
+            withConfig("regions", GravityFlipConfig.CODEC);
+
     public GravityFlipPlugin(JavaPluginInit init) {
         super(init);
     }
 
     @Override
     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).
         getLogger().at(Level.INFO).log("Gravity Flip enabled");
     }
 
     @Override
     protected void shutdown() {
+        // 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");
     }
+
+    /**
+     * Accessor for the region config holder. <strong>SAVE CONTRACT:</strong> any
+     * caller that mutates {@code configHolder().get().getRegions()} MUST call
+     * {@code configHolder().save()} afterwards. There is no lifecycle hook that
+     * auto-saves on shutdown. {@code Config.get()} returns a SHARED MUTABLE
+     * reference; concurrent writers corrupt state — Phase 02-02's
+     * {@code RegionRegistry} snapshots it into an {@code AtomicReference} for
+     * tick-loop reads.
+     */
+    public Config<GravityFlipConfig> configHolder() {
+        return configHolder;
+    }
 }