refactor(ffi-linkage): Phase 8 pre-cutover — migrate identity ffi-foreign-* test decls

Migrate the DECLS of the 7 identity-#foreign feature tests to extern/export
(1205-global/-helper, 1207, 1218-cvariadic, 1219, 1306, 1318): fn/global markers →
extern, the 2 objc import classes (1306/1318) → postfix '#objc_class("X") extern {'.
Behavior-preserving (A→B gate + existing extern twins guarantee identical output);
empty snapshot diff, corpus-validated. Comment-only #foreign in these files is left
for the Phase 9.3 doc/comment purge (comments aren't parsed → not cutover-critical).
Suite green (647 corpus / 444 unit, 0 failed).

examples/1306-ffi-objc-foreign-class-chained-dispatch.sx

@@ -12,12 +12,12 @@
 #import "modules/std.sx";
 #import "modules/build.sx";
 
-NSObject :: #foreign #objc_class("NSObject") {
+NSObject :: #objc_class("NSObject") extern {
     alloc :: () -> *NSObject;
     init :: (self: *Self) -> *Self;
 }
 
-NSObjectSelfReturn :: #foreign #objc_class("NSObject") {
+NSObjectSelfReturn :: #objc_class("NSObject") extern {
     alloc :: () -> *Self;
     init :: (self: *Self) -> *Self;
 }