Keep several saves, with migration, atomic writes and import previews (Closes #37)

Savegames now lists independent save slots made with the loaded ROM:
new game, continue, rename, duplicate, export and delete. The native
database moves to IndexedDB version 2; the singleton save becomes
"Save 1" inside the upgrade transaction, so it migrates exactly once and
the old store is left untouched. Each write adds a generation and drops
older ones in one transaction, keeping two records with CRC-32
checksums: a damaged newest record falls back to the previous one.
Slots and records from a newer player are kept and never written.

Imports are checked in a scratch world before anything changes and
previewed with the trainer, progress, saved time, game data version and
the slots of the same trainer; they go into a new save or replace one,
whose previous record stays recoverable.

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

src/game/world/world.ts

@@ -63,6 +63,7 @@ import {
   type DayTimerName,
   pageCues,
   type SoundCue,
+  landmarkName,
 } from "../scripts/interpreter";
 import { initialMagikarpRecord, type MagikarpRecord } from "../scripts/lake-of-rage";
 import {
@@ -126,6 +127,7 @@ import {
   type BattleSound,
 } from "../battle/battle";
 import { C } from "../battle/constants";
+import type { SlotSummary } from "../../storage/slots";
 import {
   DIRECTION_ORDER,
   VECTORS,
@@ -845,6 +847,11 @@ export class GameWorld {
     this.initCallReceiveDelay();
   }
 
+  /** A new world on the same ROM, clock and random source. */
+  fresh() {
+    return new GameWorld(this.assets, this.now, this.random);
+  }
+
   get x() {
     return this.player.x;
   }
@@ -4886,6 +4893,28 @@ export class GameWorld {
 
   // ---------------------------------------------------------------- saves
 
+  /** The Savegames list entry for a save: the trainer, the landmark (the
+   * backup map's in LANDMARK_SPECIAL rooms), play time, Johto badges and
+   * caught count. */
+  describe(save: NativeSave = this.save()): SlotSummary {
+    let landmark = this.assets.map(save.map).landmark;
+    if (landmark === OW.LANDMARK_SPECIAL && save.backupWarp)
+      landmark = this.assets.map(save.backupWarp.map).landmark;
+    let badges = 0;
+    for (let bit = 0; bit < OW.NUM_JOHTO_BADGES; bit++)
+      if (save.flags.includes(OW.ENGINE_ZEPHYRBADGE + bit)) badges++;
+    return {
+      player: save.name,
+      playerId: save.playerId,
+      place: landmarkName(this.assets.reader, landmark, this.host(PLAYER).text()),
+      hours: save.gameTime.hours,
+      minutes: save.gameTime.minutes,
+      badges,
+      caught: save.pokedex.caught.length,
+      newGame: save.intro,
+    };
+  }
+
   save(): NativeSave {
     // Incomplete interactions are replayed after reload; no partial quest writes.
     // The play time keeps counting through scenes, like GameTimer.