turbo-tasks: migrate TaskStorage call sites to TaskStorageBox
Preparation for step 3 (inlining the lazy-tail byte buffer into
TaskStorageBox's allocation): after step 3, stack-allocated TaskStorage
values can't grow their tail because the allocation context belongs to
TaskStorageBox. Migrate every TaskStorage::new() / TaskStorage::default()
call site to TaskStorageBox::new() so no test or production path ends up
trying to mutate a stack-allocated TaskStorage.
Specific changes:
* All 20+ test sites in storage_schema.rs switch to TaskStorageBox::new().
Access to inline fields goes through Deref/DerefMut.
* Macro-emitted `clone_snapshot()` now returns TaskStorageBox.
* Macro-emitted `restore_from` / `restore_meta_from` / `restore_data_from`
take TaskStorageBox by value (formerly TaskStorage).
* `restore_meta_from` / `restore_data_from` made `pub` (previously
`pub(crate)`) for the tests that drive them directly.
* `BackingStorage::lookup_data` / `batch_lookup_data` (and the `Either`
forwarding impl) switch their TaskStorage parameter / return types to
TaskStorageBox. `kv_backing_storage` and `restore_task_data` /
`restore_task_data_batch` follow.
* `TaskRestoreEntry::{data,meta}_restore_result` and the
`apply_restore_result` parameter shape switch to TaskStorageBox.
* The verify_serialization round-trip in `encode_task_data_inner` now
builds via TaskStorageBox::new().
* Storage snapshot path drops the wrapper that previously did
`TaskStorageBox::from_boxed(Box::new(clone_snapshot()))` — clone_snapshot
returns the right type now.
The macro still emits a public `TaskStorage::new()` because the body of
TaskStorageBox::new() needs to construct an underlying TaskStorage; that
function is left in place for now and will be redesigned in step 3 when
the underlying allocation switches to one unified head+tail buffer.
50 backend tests pass; whole workspace builds.
