enforce empty @externals

packages/emitter/src/decorators.ts

···
     });
     return;
   }
+
   context.program.stateSet(externalKey).add(target);
 }
 

packages/emitter/src/emitter.ts

···
 
     // Skip external namespaces - they don't emit JSON files
     if (isExternal(this.program, ns)) {
+      // Validate that all models in external namespaces are empty (stub-only)
+      for (const [_, model] of ns.models) {
+        if (model.properties && model.properties.size > 0) {
+          this.program.reportDiagnostic({
+            code: "external-model-not-empty",
+            severity: "error",
+            message: `Models in @external namespaces must be empty stubs. Model '${model.name}' in namespace '${fullName}' has properties.`,
+            target: model,
+          });
+        }
+      }
       return;
     }
 

packages/emitter/test/spec/external/input/test/external.tsp

···
 
 @external
 namespace test.external {
-  model Main {
-    shouldNotEmit: string;
-  }
+  model Main { }
 
-  model AlsoNotEmitted {
-    @required
-    value: boolean;
-  }
+  model AlsoNotEmitted { }
 }