world: resolve the region directory through the server, not by probing

Paper 26.x stores every dimension under <main>/dimensions/<ns>/<path>/,
and the directory does not exist until the first save, so a brand-new world
was reported as "no region directory found" and never tracked. Ask
DimensionType.getStorageFolder for the exact path and accept that it may not
exist yet; the legacy layouts are still probed for migrated servers.

The seeder now treats a missing directory as an empty world.

Claude-Session: https://claude.ai/code/session_011FePLXwBsCGLTzaSkk1Z6V

src/main/java/lv/janhouse/mapsyncer/discovery/DiscoverySeeder.java

@@ -45,6 +45,10 @@ public final class DiscoverySeeder {
 
     public Result seed(TrackedWorld world, long minInhabitedTicks, Consumer<String> log) {
         List<Path> files = new ArrayList<>();
+        if (!Files.isDirectory(world.regionDir())) {
+            // Brand-new world: nothing has been saved yet.
+            return new Result(0, 0, 0, false);
+        }
         try (DirectoryStream<Path> stream = Files.newDirectoryStream(world.regionDir(), "r.*.mca")) {
             for (Path p : stream) {
                 files.add(p);

src/main/java/lv/janhouse/mapsyncer/world/TrackedWorld.java

@@ -63,20 +63,34 @@ public final class TrackedWorld {
     // ---------------------------------------------------------------- layout
 
     /**
-     * Locates the {@code region/} directory. Paper 26.x keeps every dimension,
-     * the main overworld included, under {@code <main>/dimensions/<ns>/<path>/};
-     * older layouts are probed after that so a migrated server still works.
+     * Locates the {@code region/} directory.
+     *
+     * <p>Asks the server for the dimension's storage folder (Paper 26.x keeps
+     * every dimension, the main overworld and Bukkit's extra worlds included,
+     * under {@code <main>/dimensions/<ns>/<path>/}). The directory may not exist
+     * yet for a brand-new world; that is fine, it simply has no region files.
+     * Older layouts are probed only if the server's answer does not exist and
+     * one of them does, so a migrated server keeps working.
      */
     static Path findRegionDir(World world) {
+        List<World> worlds = Bukkit.getWorlds();
+        Path main = worlds.isEmpty() ? world.getWorldFolder().toPath() : worlds.get(0).getWorldFolder().toPath();
+        Path nms = null;
+        try {
+            var level = ((org.bukkit.craftbukkit.CraftWorld) world).getHandle();
+            nms = net.minecraft.world.level.dimension.DimensionType.getStorageFolder(level.dimension(), main)
+                    .resolve("region");
+        } catch (RuntimeException | LinkageError e) {
+            // Fall through to probing.
+        }
+        if (nms != null && Files.isDirectory(nms)) {
+            return nms;
+        }
         List<Path> candidates = new ArrayList<>();
         String ns = world.getKey().getNamespace();
         String path = world.getKey().getKey();
-        List<World> worlds = Bukkit.getWorlds();
-        Path main = worlds.isEmpty() ? null : worlds.get(0).getWorldFolder().toPath();
         Path own = world.getWorldFolder().toPath();
-        if (main != null) {
-            candidates.add(main.resolve("dimensions").resolve(ns).resolve(path).resolve("region"));
-        }
+        candidates.add(main.resolve("dimensions").resolve(ns).resolve(path).resolve("region"));
         candidates.add(own.resolve("dimensions").resolve(ns).resolve(path).resolve("region"));
         switch (world.getEnvironment()) {
             case NETHER -> candidates.add(own.resolve("DIM-1").resolve("region"));
@@ -89,7 +103,7 @@ public final class TrackedWorld {
                 return c;
             }
         }
-        return null;
+        return nms != null ? nms : candidates.get(0);
     }
 
     // ---------------------------------------------------------------- layers