The entire surface of `datavolve`: one factory, three builder members, and the result types they produce.

## `datavolve()`

Creates an empty, immutable datavolve instance.

### `.add(version, evolve)`

Appends an evolution and returns a **new** datavolve instance whose output type is inferred from `evolve`'s return value.

- `version` - a positive integer, sequential from `1`. Adding a non-sequential version throws immediately (a build-time programmer error), which also protects you from silently misaligning stored data if an evolution is deleted.
- `evolve` - `(prev) => next`. `prev` is `unknown` for version `1`, and the previous evolution's output type thereafter.

### `.run(data, fromVersion)`

Runs the applicable evolutions. **Total** - it never throws; an evolution that itself throws is caught and reported. Returns an `EvolveResult`.

- `data: unknown` - the stored value.
- `fromVersion: number` - the version `data` is currently at. Required.

### `.latestVersion`

The highest version datavolve knows about. Use it to stamp data on write-back:

```typescript
storage.setItem(
  key,
  JSON.stringify({ version: settings.latestVersion, value }),
);
```

## Types

```typescript
type EvolveResult<T> =
  | { success: true; value: T; version: number } // version = the version `value` is now at (always latestVersion)
  | { success: false; error: EvolveError };

type EvolveError =
  | { code: "ahead"; fromVersion: number; latestVersion: number }
  | { code: "malformed"; fromVersion: number }
  | { code: "failed"; failedVersion: number; cause: unknown };

type Datavolve<T>; // the datavolve instance, generic over its latest output type
```

## Version Behavior

| `fromVersion`                     | Result                                                              |
| --------------------------------- | ------------------------------------------------------------------- |
| `=== latestVersion`               | success, value returned unchanged (no-op fast path)                 |
| `0 ≤ fromVersion < latestVersion` | success, evolutions `(fromVersion, latestVersion]` applied in order |
| `> latestVersion`                 | failure `ahead`                                                     |
| `< 0` or non-integer              | failure `malformed`                                                 |
| an evolution throws               | failure `failed` (with `cause`)                                     |