createLocationState: option to drop state written with an older shape (version / validate) #994

Problem

A createLocationState key is derived from the file path and export name (src/vite/plugins/expose-ids/handler-transform.ts:48-53), so it stays the same across deploys. The read is an unchecked cast (src/browser/react/location-state.ts:20).

A tab that stays open across a deploy that changed the state's shape restores the old value typed as the new one. Code then reads missing or renamed fields, or a changed structure, with no type error and no runtime check. History entries survive reloads and back/forward indefinitely, so long-lived tabs hit this after an ordinary release.

Seen on 0.18.0.

Proposal

An option on createLocationState that lets a definition reject state it can't trust, for example:

export const GridState = createLocationState<GridSnapshot>({ version: 2 });
// stored as { v: 2, value }; a different or missing version reads as undefined

export const GridState = createLocationState<GridSnapshot>({
  validate: (value): value is GridSnapshot => isGridSnapshot(value),
});
// run on read; a failing value reads as undefined

version is cheap and covers deliberate shape changes. validate also covers state written by other code under the same key. Either keeps the typed read honest. An entry that fails the check reads as undefined, which is the same result as no state.