refactor(ffi-linkage): Phase 9.2b-fix — use is_extern (not new is_reference) for the runtime-class ref flag

Per user feedback: don't introduce new terminology. The RuntimeClassDecl
reference-vs-define flag (set by the postfix 'extern' modifier, == old prefix
'#foreign #objc_class') is named is_extern, matching the keyword that drives it
and the existing is_extern on VarDecl/IR. Renamed is_reference→is_extern,
is_reference_eff→is_extern_eff; updated the field comment. Snapshot-neutral; green.

src/ir/lower/objc_class.zig

@@ -139,7 +139,7 @@ pub fn lookupObjcDefinedStateFieldOnPointer(self: *Lowering, obj_expr: *const as
     const fcd = self.program_index.runtime_class_map.get(struct_name) orelse return null;
     // Only sx-defined Obj-C classes have a state struct. Foreign
     // classes' fields are purely declaration metadata (no state).
-    if (fcd.is_reference or fcd.runtime != .objc_class) return null;
+    if (fcd.is_extern or fcd.runtime != .objc_class) return null;
     // Skip property fields — those dispatch via the M2.2 getter/setter
     // path. Plain instance fields take the ivar+gep path.
     for (fcd.members) |m| switch (m) {