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/program_index.test.zig

@@ -73,7 +73,7 @@ test "ProgramIndex declaration maps round-trip (A1.1b)" {
         .foreign_path = "NSString",
         .runtime = .objc_class,
         .members = &.{},
-        .is_reference = true,
+        .is_extern = true,
         .is_main = false,
     };
     try idx.runtime_class_map.put("NSString", &fcd);