refactor(ffi-linkage): Phase 7.3 — migrate 14xx ffi-jni examples #foreign→extern

13 JNI examples migrated (1410-1419/1423/1424/1425): import runtime classes
'#foreign #jni_class("X") {' → '#jni_class("X") extern {'. 1417 (all-runtimes)
also exercises #jni_interface/#objc_class/#objc_protocol/#swift_class/#swift_struct/
#swift_protocol — all take the postfix modifier (verified by probe), migrated via a
generalized '#foreign #<directive>("X") {' → '… extern {' rewrite. No 14xx snapshot
asserts on 'foreign'; empty snapshot diff, corpus-validated.

KEPT comment-only #foreign in 1426 (jni-extern-class test, no decls). Suite green
(647 corpus / 444 unit, 0 failed).

examples/1412-ffi-jni-class-02-method.sx

@@ -10,7 +10,7 @@
 
 // `Self` here refers to the enclosing `View` type — resolved at
 // 2.x sema, not at parse time.
-View :: #foreign #jni_class("android/view/View") {
+View :: #jni_class("android/view/View") extern {
     getId :: (self: *Self) -> i32;
 }