objc: migrate remaining ns_string call sites to xx NSString

NSLog's fmt, addObserver's name, UIApplicationMain's principal-class, CADisplayLink's run-loop mode, and metal's newLibraryWithSource/newFunctionWithName string args are retyped *NSString, so their call sites read xx "..." instead of ns_string("...".ptr). ns_string is now used only by impl Into(*NSString) for string.

library/modules/std/objc.sx

@@ -72,7 +72,7 @@ objc_registerClassPair :: (cls: *void)                                        ->
 
 // Foundation C-API helpers (Foundation is already linked above via #framework).
 // NSLog takes an NSString format; the variadic tail is not exposed here.
-NSLog :: (fmt: *void) #foreign;
+NSLog :: (fmt: *NSString) #foreign;
 
 // ─── Convenience helpers ────────────────────────────────────────────────
 // These hide the typed-fn-pointer cast for the most common shapes. They