fix: _setjmp redeclared with mismatched signatures in gc.rs vs promise.rs (potential UB) #856

cargo build --release flags a clashing extern declaration for _setjmp:

warning: `setjmp` redeclares `_setjmp` with a different signature
    --> crates/perry-runtime/src/promise.rs:956:9
     |
 956 |         #[link_name = "_setjmp"]
     |         ^^^^^^^^^^^^^^^^^^^^^^^^ this signature doesn't match the previous declaration
     |
    ::: crates/perry-runtime/src/gc.rs:1860:13
     |
1860 |             #[link_name = "_setjmp"]
     |             ------------------------ `_setjmp` previously declared here
     |
     = note: expected `unsafe extern "C" fn(*mut u64) -> i32`
                found `unsafe extern "C" fn(*mut i32) -> i32`
     = note: `#[warn(clashing_extern_declarations)]` on by default

Why this matters

Both sites link to the same C symbol (_setjmp) with different parameter types (*mut u64 vs *mut i32). If they happen to be called with buffers sized for one but the runtime layout matches the other, this is undefined behavior — buffer corruption is silent and platform-dependent. The fact that it currently appears to work is luck of the macOS / aarch64 ABI, not safety.

Investigation needed

1. What does _setjmp actually take on macOS / Linux?
   • On darwin, jmp_buf is int[_JBLEN] — i.e. effectively *mut c_int.
   • On glibc Linux, the layout is also int[] though _JBLEN differs.
2. What does each call site actually use the buffer for?
   • gc.rs uses it as a register-snapshot blob (u64[]).
   • promise.rs uses it for an unwind context (i32[]).
3. Are either of them allocating the wrong-sized buffer?

Proposed fix shape

Introduce a single extern "C" block (e.g. runtime::ffi::setjmp or as a small shared module) with the correct libc-matching signature. Have both gc.rs and promise.rs import that one declaration. Remove the duplicated #[link_name] declarations.

If the two sites genuinely need to view the same buffer through different lenses, cast the pointer at the call site rather than redeclaring the extern.

Context

Spotted by the warning-cleanup PR (chore: reduce compile warnings 325 → 125). Kept out of that PR because it requires investigation of libc layouts and the runtime's stack-unwinding contract.