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/sema.zig

@@ -1334,14 +1334,14 @@ pub const Analyzer = struct {
             },
             .runtime_class_decl => |fd| {
                 try self.addSymbol(fd.name, .type_alias, null, node.span);
-                if (fd.is_reference and fd.is_main) {
+                if (fd.is_extern and fd.is_main) {
                     try self.diagnostics.append(self.allocator, .{
                         .level = .err,
                         .message = "'#foreign' and '#jni_main' / '#objc_main' are mutually exclusive — a foreign-referenced class can't be the app's main entry",
                         .span = node.span,
                     });
                 }
-                if (fd.is_reference) {
+                if (fd.is_extern) {
                     for (fd.members) |m| switch (m) {
                         .method => |md| if (md.body != null) {
                             try self.diagnostics.append(self.allocator, .{