objc: NSString type + Into(*NSString) for string

Adds an NSString foreign class and impl Into(*NSString) for string so a string literal flows into any *NSString slot via xx. uikit's keyboard userInfo lookups now read objectForKey(xx "...") instead of ns_string("...".ptr), and objectForKey's key param is retyped *NSString.

ffi-objc-call-06 .ir snapshot regenerated: declaring the NSString type adds its reflection thunks (struct_to_string/pointer_to_string), same as the existing NSObject/NSDictionary. Runtime output unchanged.

library/modules/std/objc.sx

@@ -123,6 +123,24 @@ NSObject :: #foreign #objc_class("NSObject") {
     respondsToSelector :: (self: *Self, sel: *void) -> BOOL;
 }
 
+// ─── NSString ────────────────────────────────────────────────────────────
+// Foundation's immutable string. `ns_string` builds an autoreleased instance
+// from a C string; the `Into` impl lets a string literal flow into any
+// `*NSString` slot via `xx`, e.g. `dict.objectForKey(xx "SomeKey")`.
+NSString :: #foreign #objc_class("NSString") {
+    #extends NSObject;
+    UTF8String :: (self: *Self) -> [*]u8;
+}
+
+// `self.ptr` must be NUL-terminated. String literals are; an arbitrary
+// substring/built `string` may not be, so only pass literals (or otherwise
+// NUL-terminated slices) through this conversion.
+impl Into(*NSString) for string {
+    convert :: (self: string) -> *NSString {
+        return xx ns_string(self.ptr);
+    }
+}
+
 // ─── Autoreleasepool (M4.A) ──────────────────────────────────────────────
 // Foundation factory methods (`NSString.stringWithUTF8String:`,
 // `[NSArray array]`, ...) return autoreleased objects — valid until the