turbo-tasks: rename TaskStorageHead → TaskStorageInline (and .head → .inline)
The sub-struct that groups TaskStorage's inline fields is no longer "the
head" in any meaningful sense — `TaskStorageInner` itself is the head of
the unified head+tail allocation, and the sub-struct is just an
organizational grouping of fields marked `inline` in the schema.
`TaskStorageHead` → `TaskStorageInline`. The corresponding field
`TaskStorageInner.head: TaskStorageHead` becomes
`TaskStorageInner.inline: TaskStorageInline`. The macro-generated
inline-field accessors switch from `self.head.<field>` to
`self.inline.<field>` access patterns.
The `TaskStorage::head()` helper method (returning `&TaskStorageInner`)
keeps its name — `head` is correct there: it points at the head bytes of
the unified allocation, as distinct from `tail_ptr`.
50 unit tests pass; integration test suite passes; whole workspace builds.