[turbopack] Simplify snapshotting logic (#91178)
## Summary
- **Use `modified` flags instead of `restored` flags** to decide which task data categories to encode during snapshots. Previously, `process` used `restored` flags (encoding any category that had been loaded into memory), while `process_snapshot` used `modified` flags. The `restored` check was overly conservative — it would re-serialize categories that were loaded from disk but never actually changed. Using `modified` for both paths only encodes categories that were actually dirtied, avoiding redundant re-persistence of clean data.
- **Unify `process` and `process_snapshot` into a single callback** on `take_snapshot`. Since both closures now use the same logic (`modified` flags), the two-callback API was unnecessary. `take_snapshot` now takes a single `Fn(TaskId, &TaskStorage, &mut TurboBincodeBuffer) -> SnapshotItem` — for snapshot copies the `Box<TaskStorage>` is simply dereferenced before calling.
- **Change transient task handling from silent empty return to `unreachable!`**, since transient tasks should never appear in the persistence path.
## Test plan
- Existing snapshot/persistence tests should continue to pass
- The `modified` flags are always set for any task that ends up in the modified map (via `track_modification_internal`), so no categories are missed
