Add the Johto and Kanto atlas to the Map tab

The Map tab draws the region's Town Map from the ROM with the player's
landmark, the visited flypoints as Fly destinations and the landmarks
whose maps are ported. Kanto opens with the Fly map's Indigo Plateau
rule. Landmark positions and flypoints are compared with the pinned
PRET source.

Closes #36

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/game/rom/assets.ts

@@ -33,6 +33,7 @@ export class CrystalAssets {
   private tilesets = new Map<string, Tileset>();
   private sprites = new Map<number, Sprite>();
   private spawnFlags?: Map<number, number>;
+  private openLandmarks?: Set<number>;
   private sceneOwners?: Map<string, string>;
   constructor(bytes: Uint8Array) {
     this.reader = new RomReader(bytes);
@@ -439,6 +440,13 @@ export class CrystalAssets {
   }
   /** The engine flag (ENGINE_FLYPOINT_*) of a wVisitedSpawns bit, from the
    * ROM's EngineFlags; undefined for spawn points without one. */
+  /** The landmarks with at least one ported map. */
+  portedLandmarks(): ReadonlySet<number> {
+    this.openLandmarks ??= new Set(
+      Object.keys(NATIVE_MAPS).map((id) => this.map(id).landmark),
+    );
+    return this.openLandmarks;
+  }
   spawnEngineFlag(spawn: number): number | undefined {
     if (!this.spawnFlags) {
       const r = this.reader;