[BUG] Crash (`RuntimeError: unreachable`) when opening existing `idb://` database created by previous pglite version

### Description

We’re seeing a hard WASM crash when pglite tries to open an existing IndexedDB-backed database that was created by an earlier pglite / wasm build.

Instead of a recoverable error, the runtime aborts with:

```
RuntimeError: unreachable
at pglite.wasm:0x5154f8
…
```

This happens when:

* a database already exists in `idb://`
* pglite or its wasm build is upgraded (or extensions change)
* the same `dataDir` is reused

Creating the same database under a **new name** works immediately, which strongly suggests an on-disk compatibility issue.

### Version

Our currently used version is: `0.3.14`

### Expected behavior

Ideally one of:

* a clear error indicating an incompatible database format/version, or
* a guard that refuses to open incompatible IDB data instead of crashing the WASM runtime.

### Actual behavior

* WASM traps with `unreachable`
* No JS-level error is catchable
* Application crashes during worker init

### Notes

* This is reproducible without changing application logic.
* The issue also appears more frequently when using extensions (e.g. `vector`) and `idb://`.
* As a workaround, we are versioning the database name (`idb://name_vX`) and treating IDB storage as ephemeral.

### Suggestion

It would be very helpful if pglite could:

* embed a format / build version marker in IDB metadata, and
* fail fast with a descriptive error when opening incompatible data.