Function lumen_signal_set_int64 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn lumen_signal_set_int64(
    name: *const c_char,
    value: i64,
) -> LumenStatus

Set a scalar signal to a 64-bit signed integer, typed.

Pushes a PropertyValue::I64 through the foundation typed-property bus so the receiving cell in PropertyStore keeps the typed variant (no stringify-on-write, no parse-on-read). Mirrors the write into the FFI-local cache for pre-run read-back.