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>

tests/human-test-saves.test.ts

@@ -8,9 +8,9 @@ import { encodeSave } from "../src/storage/saves";
 import { foundationFixture } from "./foundation-fixture";
 
 // Backups for the manual checks on the project board's "A tester" column,
-// built from the local ROM and imported through Savegames → Import native
-// backup. They hold names read from the ROM, so they stay out of the
-// repository (*.crystal-save.json is ignored).
+// built from the local ROM and imported through Savegames → Import backup
+// (then "Add as a new save"). They hold names read from the ROM, so they
+// stay out of the repository (*.crystal-save.json is ignored).
 //
 //   MAKE_TEST_SAVES=1 npx vitest run tests/human-test-saves.test.ts
 const romPath =