refactor: Rust 2024 — fix static_mut_refs and migrate deprecated libc::mach_host_self

[TheHypnoo]

Three warnings flag forward-compatibility / dependency migration work.

1. static_mut_refs (2 sites)

Both create a shared reference to a mutable static. In Rust 2024 this becomes a hard error.

warning: creating a shared reference to mutable static
   --> crates/perry-runtime/src/object.rs:796:22
    |
796 |         for entry in TRANSITION_CACHE_GLOBAL.iter() {
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ shared reference to mutable static

warning: creating a shared reference to mutable static
    --> crates/perry-runtime/src/string.rs:2828:22
     |
2828 |         for entry in INTERN_TABLE.iter() {
     |                      ^^^^^^^^^^^^^^^^^^^ shared reference to mutable static

Fix options:

a) Wrap the static in OnceCell / Mutex / RwLock — correct, but adds locking on hot paths (transition cache, string intern).
b) Use &raw const to obtain a raw pointer and iterate via unsafe — preserves performance, makes the unsafety explicit.

The right call depends on whether either path needs concurrent access; both currently look thread-local. Recommend option (b) unless a thread-safety audit shows otherwise.

2. Deprecated libc::mach_host_self (1 site)

warning: use of deprecated function `libc::mach_host_self`: Use the `mach2` crate instead

Requires adding the mach2 dependency and updating the call site.

Why grouped

All three are "prepare for Rust 2024 / current deprecations". Same reviewer mental model. If the mach2 migration adds non-trivial API surface or new dependency tree, split into 3a (static_mut_refs) and 3b (mach2 migration).

Context

Carved out of the warning-cleanup PR (chore: reduce compile warnings 325 → 125).