Fix AllowTrackedCall test: make it self-contained with untracked lambda

The test was referencing Unsafe.unsafe_untracked_call from the prelude,
but test .sk files are compiled standalone. Define @allow_tracked_call
locally and use `untracked () -> ...` lambda syntax.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: mbouaziz

skiplang/compiler/tests/Typechecking/AllowTrackedCall.sk

@@ -1,14 +1,19 @@
 module TypecheckingAllowTrackedCall;
 // @allow_tracked_call lets a tracked function call an untracked function
 
+@allow_tracked_call
+untracked fun unsafeUntrackedCall<T>(f: untracked () -> T): T {
+  f()
+}
+
 untracked fun sideEffect(): String {
   "OK"
 }
 
 // This function is tracked (default), but can call sideEffect()
-// via Unsafe.unsafe_untracked_call
+// via unsafeUntrackedCall which has @allow_tracked_call
 fun trackedCaller(): String {
-  Unsafe.unsafe_untracked_call(() -> sideEffect())
+  unsafeUntrackedCall(untracked () -> sideEffect())
 }
 
 untracked fun main(): void {