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/1318-ffi-objc-property-foreign.sx

@@ -31,7 +31,7 @@ probe_set_tag :: (self: *void, _cmd: *void, v: i32) callconv(.c) {
 }
 
 // Foreign declaration with #property on `tag`.
-SxPropProbe :: #foreign #objc_class("SxPropProbe") {
+SxPropProbe :: #objc_class("SxPropProbe") extern {
     alloc :: () -> *SxPropProbe;
     init  :: (self: *SxPropProbe) -> *SxPropProbe;
     tag: i32 #property;