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/1411-ffi-jni-class-01-empty.sx

@@ -1,5 +1,5 @@
 // Phase 2 step 2.0 (PLAN-FFI.md): xfail parser test for the
-// type-introducer `Foo :: #foreign #jni_class("java/path/Foo") { }` directive.
+// type-introducer `Foo :: #jni_class("java/path/Foo") extern { }` directive.
 //
 // Today's parser doesn't recognize `#jni_class` as a known hash
 // directive after `::`, so it falls through to expression parsing
@@ -10,7 +10,7 @@
 
 #import "modules/std.sx";
 
-Foo :: #foreign #jni_class("java/path/Foo") { }
+Foo :: #jni_class("java/path/Foo") extern { }
 
 main :: () -> i32 {
     print("parse-only ok\n");