refactor(command): simplify region lookup in GravityFlip commands

- Replaced manual region search loops with a new `find` method in RegionRegistry for improved readability and efficiency.
- Updated `GravityFlipToggleSubCommand` and `GravityFlipTpSubCommand` to utilize the new method for finding regions by name.
- Enhanced the `add` method in RegionRegistry to use the `find` method for checking existing region names.

src/main/java/com/mythlane/gravityflip/region/RegionRegistry.java

@@ -78,13 +78,20 @@ public final class RegionRegistry {
         return out;
     }
 
+    /** Returns the region with the given name from the current snapshot, or null. */
+    public GravityFlipRegion find(String name) {
+        if (name == null) return null;
+        for (GravityFlipRegion x : regionsSnapshot.get()) {
+            if (x.getName().equals(name)) return x;
+        }
+        return null;
+    }
+
     /** Adds a region; throws IllegalArgumentException if the name already exists. */
     public void add(GravityFlipRegion r) {
         synchronized (mutationLock) {
-            for (GravityFlipRegion x : regionsSnapshot.get()) {
-                if (x.getName().equals(r.getName())) {
-                    throw new IllegalArgumentException("region name already exists: " + r.getName());
-                }
+            if (find(r.getName()) != null) {
+                throw new IllegalArgumentException("region name already exists: " + r.getName());
             }
             config.getRegions().add(r);
             regionsSnapshot.set(List.copyOf(config.getRegions()));