BridgeJS: Diagnose async and throwing @JS struct initializers

Plugins/BridgeJS/Sources/BridgeJSCore/SwiftToSkeleton.swift

@@ -1986,6 +1986,14 @@ private final class ExportSwiftAPICollector: SyntaxAnyVisitor {
             guard let effects = collectEffects(signature: node.signature) else {
                 return .skipChildren
             }
+            if effects.isAsync || effects.isThrows {
+                diagnose(
+                    node: node,
+                    message: "@JS struct initializer cannot be async or throw",
+                    hint: "BridgeJS calls this initializer to rebuild struct values passed from JavaScript"
+                )
+                return .skipChildren
+            }
 
             let structAbiName = exportedStructByName[structKey]?.abiName ?? "unknown"
             let constructor = ExportedConstructor(

Plugins/BridgeJS/Tests/BridgeJSToolTests/DiagnosticsTests.swift

@@ -535,6 +535,21 @@ import Testing
         #expect(skeleton.exported != nil)
     }
 
+    @Test
+    func throwingStructInitProducesDiagnostic() throws {
+        let source = """
+            @JS struct Point {
+                var x: Double
+
+                @JS init(x: Double) throws(JSException) {
+                    self.x = x
+                }
+            }
+            """
+        let diagnostics = try #require(moduleDiagnostics(source: source))
+        #expect(diagnostics.description.contains("@JS struct initializer cannot be async or throw"))
+    }
+
     @Test
     func structWithoutExplicitInitSucceeds() throws {
         let source = """