refactor(ffi-linkage): Phase 7.2 — migrate 13xx ffi-objc examples #foreign→extern

18 obj-c examples migrated (1308/1311-1317/1319/1320/1321/1341-1347): import
runtime classes '#foreign #objc_class("X") {' → '#objc_class("X") extern {'
(prefix→postfix) + fn/comment '#foreign'→'extern'. No 13xx snapshot asserts on
'foreign' text → all behavior-preserving; empty snapshot diff, corpus-validated.

Per the keep-list policy: KEPT identity-#foreign tests 1306/1318 (filename
ffi-*-foreign*); LEFT comment-only #foreign in the extern/export test files
1332/1348/1349 (no decls). Bare defined #objc_class examples (no #foreign) untouched
— not a purge target. Suite green (647 corpus / 444 unit, 0 failed).

examples/1317-ffi-objc-class-level-constant.sx

@@ -14,7 +14,7 @@
 #import "modules/build.sx";
 #import "modules/ffi/objc.sx";
 
-NSObject :: #foreign #objc_class("NSObject") {
+NSObject :: #objc_class("NSObject") extern {
     alloc :: () -> *NSObject;
     init  :: (self: *NSObject) -> *NSObject;
 }