Transformer: Map initialization patterns to getOrInsert APIs

## Summary
Add a transformer that upgrades common Map get-or-init patterns to getOrInsert/getOrInsertComputed.

## Examples
```js
// before
if (!cache.has(key)) {
cache.set(key, []);
}
const value = cache.get(key);

// after
const value = cache.getOrInsert(key, []);
```

```js
// before
if (!cache.has(key)) {
cache.set(key, buildValue());
}
const value = cache.get(key);

// after
const value = cache.getOrInsertComputed(key, buildValue);
```

## Scope
- Baseline category: newly available.
- Preserve side-effect behavior; skip unsafe cases.

## Acceptance criteria
- Implement in src/newlyAvailable/
- Export from src/newlyAvailable.js
- Add tests in tests/newlyAvailable/
- Document in README.md