turbo-tasks: route TaskStorageAccessors through TaskStorageBox
`typed()` / `typed_mut()` on the `TaskStorageAccessors` trait now return
`&TaskStorageBox` / `&mut TaskStorageBox` rather than the inner
`&TaskStorage` / `&mut TaskStorage`. `TaskStorageBox` implements
`Deref<Target = TaskStorage>`, so every existing macro-emitted callsite
of the form `self.typed().<field>()` keeps working unchanged — the
inline-field and lazy-field accessors are still defined on `TaskStorage`
and reached via deref coercion.
Why: step 3 of the refactor will inline `LazyTail`'s byte buffer into
`TaskStorageBox`'s heap allocation, and the grow-capable lazy mutations
(currently `LazyTail::install` on `&mut LazyTail`) need access to the
owning pointer to relocate the buffer. Returning the box from
`typed_mut()` is the seam those grow paths will hook into.
`StorageWriteGuard::Deref::Target` flips from `TaskStorage` to
`TaskStorageBox` for the same reason. The deref chain via `RefMut`
collapses to a single explicit step.
50 backend tests pass; whole workspace builds.
