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/ast.zig

@@ -888,7 +888,7 @@ pub const RuntimeClassDecl = struct {
     foreign_path: []const u8, // directive arg: "java/path/Foo" / "NSString" / "Foundation.URL"
     runtime: RuntimeKind,
     members: []const RuntimeClassMember = &.{},
-    is_reference: bool = false, // `#foreign #...` prefix — class is provided by the foreign runtime; we only reference it
+    is_extern: bool = false, // `#objc_class(…) extern` — class is provided by the runtime; we only reference it (vs `export`, which defines + registers a new sx class)
     is_main: bool = false, // `#jni_main` / `#objc_main` — class is the launchable entry (Activity / UIApplicationDelegate / ...)
     /// True when the sx-side alias NAME was a backtick raw identifier — exempt
     /// from the reserved-type-name decl check.