Collapse NoopKvDb into an empty read-only TurboPersistence
`noop_backing_storage()` now returns the same concrete type as the real
`turbo_backing_storage()` — `KeyValueDatabaseBackingStorage<TurboKeyValueDatabase>`
— built on an empty, read-only `TurboPersistence` instance that owns no
on-disk state and never touches the filesystem.
This eliminates the `Either<TurboBackingStorage, NoopBackingStorage>`
wrappers at the napi binding and turbopack-cli call sites, removes one
generic instantiation of every kv-backing-storage helper, and replaces
the silent-drop write behavior of `NoopKvDb` with a loud bail via the
existing `read_only` guard.
`TurboPersistence::new` takes a new `empty` parameter that allocates the
block caches with zero-sized buckets, avoiding the ~2 MiB of hash-table
headroom they would otherwise reserve on the noop instance.
