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>

README.md

@@ -2,7 +2,7 @@
 
 A native web port of Pokémon Crystal, translated from [PRET's Game Boy disassembly](https://github.com/pret/pokecrystal). The game runs in TypeScript and reads graphics, maps, sprites and dialogue from the user's own ROM. No original game assets are included.
 
-**Current milestone: all of Johto, from the opening to Blackthorn City, the Rising Badge and the Battle Tower.** Play in a minimal Crystal-style interface with Game, Savegames, Map, Pokédex, Link and Friends tabs. The native canvas fills the play area, and the live local map follows your position. The Map tab also shows the Johto map with your location, the towns you can fly to and the areas open so far. Save, export and import work now. The Pokédex tab shows the Pokémon you have seen and caught. Link and Friends show their planned scope without placeholder data.
+**Current milestone: all of Johto, from the opening to Blackthorn City, the Rising Badge and the Battle Tower.** Play in a minimal Crystal-style interface with Game, Savegames, Map, Pokédex, Link and Friends tabs. The native canvas fills the play area, and the live local map follows your position. The Map tab also shows the Johto map with your location, the towns you can fly to and the areas open so far. Savegames keeps several independent playthroughs: start a new game, continue, rename, duplicate, export or delete any save, and preview a backup before importing it. The Pokédex tab shows the Pokémon you have seen and caught. Link and Friends show their planned scope without placeholder data.
 
 Walk around your home, the town and its buildings at the original walking speed. Mom gives you the Pokégear and asks the day of the week. The teacher stops you from leaving town. Your rival shoves you away from the lab window. Professor Elm introduces his errand, and you choose Chikorita, Cyndaquil or Totodile. The Pokémon's picture and stats come from your ROM, and you can nickname it. Elm's aide then gives you a Potion. These scenes run the game's own event scripts and movement data, with native choices and naming.
 
@@ -47,9 +47,9 @@ npm run dev
 
 Open the printed local URL and select your uncompressed **English US/Europe revision 1.1** ROM (`.gbc`, 2 MiB). Its checksum is verified locally. ROMs are never uploaded or stored in browser storage; select yours again after reloading to resume. A new game starts with Professor Oak's introduction, where you choose your character, set the clock and pick a name.
 
-Use onscreen controls, or focus the game screen and use arrows to move, **X** to interact and **X / Z** to continue dialogue. **Settings** changes these keys, the text size and the sound volume for this browser. In choices, **Up / Down** change the answer, **X** confirms and **Z** answers No. Sessions save every 10 seconds and when paused. Switching apps pauses play. Download a backup before clearing browser data.
+Use onscreen controls, or focus the game screen and use arrows to move, **X** to interact and **X / Z** to continue dialogue. **Settings** changes these keys, the text size and the sound volume for this browser. In choices, **Up / Down** change the answer, **X** confirms and **Z** answers No. Games save every 10 seconds and when paused, each into its own save. Switching apps pauses play. Download a backup before clearing browser data.
 
-Native saves are separate from emulator snapshots and cartridge `.sav` files. Saving during a scene keeps the progress from before it, so the scene replays after loading. The time you give Oak is kept as an offset from the device clock (the steppers start at the device's time), and the weekday you give Mom as an offset from that clock's weekday. The tile counter measures in-game movement, not real-world steps.
+Native saves are separate from emulator snapshots and cartridge `.sav` files. Each save keeps its newest and previous write; if the newest was interrupted or damaged, the previous one loads and nothing is overwritten. The single save of earlier versions becomes "Save 1" the first time this version opens it. Saves and backups from a newer version are kept as they are and refused, never rewritten. Saving during a scene keeps the progress from before it, so the scene replays after loading. The time you give Oak is kept as an offset from the device clock (the steppers start at the device's time), and the weekday you give Mom as an offset from that clock's weekday. The tile counter measures in-game movement, not real-world steps.
 
 ## Reference emulator
 
@@ -87,7 +87,7 @@ CRYSTAL_ROM_PATH=/nonexistent/crystal-ci.gbc npm run test:e2e -- --workers=2
 
 `git config core.hooksPath .githooks` keeps the forge's project board in step with commits: `Closes #N` in a message moves card N to Fait and closes it, any other `#N` moves it to En cours, and a commit on `main` without a reference adds a closed card to Fait. `[no-card]` skips a commit. See [project-board.mjs](scripts/project-board.mjs).
 
-The [full port plan](docs/PLAN.md) covers remaining game systems, save slots, the world map, Pokédex, internet Link, friends and optional mobile step support for egg hatching. See the [architecture](docs/ARCHITECTURE.md) and [routine map and verification](docs/NATIVE_PORT.md) for module boundaries and reference checks.
+The [full port plan](docs/PLAN.md) covers remaining game systems, the world map, Pokédex, internet Link, friends and optional mobile step support for egg hatching. See the [architecture](docs/ARCHITECTURE.md) and [routine map and verification](docs/NATIVE_PORT.md) for module boundaries and reference checks.
 
 References: PRET [documentation](https://pret.github.io/pokecrystal/), [wiki](https://github.com/pret/pokecrystal/wiki) and [source](https://github.com/pret/pokecrystal). The development reference uses [binjgb](https://github.com/binji/binjgb) under its [MIT license](reference/public/vendor/binjgb/LICENSE); see [provenance](reference/public/vendor/binjgb/PROVENANCE.md).