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/1321-ffi-objc-defined-class-method-self.sx

@@ -3,7 +3,7 @@
 // call site, not get truncated to i8 by the enclosing function's
 // BOOL return type. Regression locks in the
 // `resolveCallParamTypes` fix that threads foreign-class method
-// param types correctly even when the receiver is a `#foreign
+// param types correctly even when the receiver is a `extern
 // #objc_class` alias. Every probe round-trips the receiver pointer
 // — a regression would read only the low byte and the observer
 // pointer would appear as e.g. 0xC0 / 0x20 instead of its real
@@ -17,7 +17,7 @@ g_observer : *void = null;
 // Stand-in for NSNotificationCenter — we just need a foreign-class
 // method with several *void args so the call site's arg-target-type
 // resolution exercises the same path as uikit.sx's keyboard observer.
-SxIssue44Bus :: #foreign #objc_class("NSNotificationCenter") {
+SxIssue44Bus :: #objc_class("NSNotificationCenter") extern {
     defaultCenter :: () -> *SxIssue44Bus;
     addObserver_selector_name_object :: (self: *Self, observer: *void, sel: *void, name: *void, obj: *void);
 }