Add Unsafe.unsafe_untracked_call to stdlib

Provides a controlled escape hatch for calling untracked functions
from tracked contexts (e.g., performing I/O inside reactive
computations where the caller takes responsibility for correctness).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mbouaziz

skiplang/prelude/src/core/Unsafe.sk

@@ -100,4 +100,13 @@ native fun array_set_byte<T>(v: mutable Array<T>, i: Int, b: UInt8): void;
 @cpp_extern
 native fun string_ptr(v: String, i: Int): Runtime.NonGCPointer;
 
+// Calls an untracked function from a tracked context.
+// Use this to perform side effects (e.g., file I/O) inside reactive
+// computations where you take responsibility for correctness and
+// invalidation.
+@allow_tracked_call
+untracked fun unsafe_untracked_call<T>(f: untracked () -> T): T {
+  f()
+}
+
 module end;