class RestrictedState

Encapsulates a thread's restricted mode state.

The underlying state is allocated and managed entirely in Rust

(`zircon/kernel/kernel/restricted_state.rs`). In C++, `RestrictedState` is an incomplete struct

type serving as an opaque handle so that C++ code can safely pass around strongly-typed

`RestrictedState*` pointers without exposing Rust-side layout details. Lifecycle management is

handled via `RestrictedStatePtr` (`ktl::unique_ptr

<RestrictedState

, RestrictedStateDeleter>`),

which dispatches to `rust_restricted_state_destroy` upon destruction.