fix: emit diagnostic for @js mutating struct methods instead of generating broken code

Per reviewer feedback (wfltaylor): mutations to `_self` are not propagated
back to JavaScript, so the mutable-self binding approach silently discards
all mutations. Replace it with a clear compile-time diagnostic pointing users
to a value-returning redesign.

Author: shivanshu877

Plugins/BridgeJS/Sources/BridgeJSCore/ExportSwift.swift

@@ -228,24 +228,14 @@ public class ExportSwift {
             }
         }
 
-        func callMethod(methodName: String, returnType: BridgeType, isMutating: Bool = false) {
-            let (selfParam, selfExpr) = removeFirstLiftedParameter()
-            if isMutating, case .swiftStruct = selfParam.type {
-                append("var _self = \(selfExpr)")
-                generateParameterLifting()
-                let item = renderCallStatement(
-                    callee: "_self.\(raw: methodName)",
-                    returnType: returnType
-                )
-                append(item)
-            } else {
-                generateParameterLifting()
-                let item = renderCallStatement(
-                    callee: "\(raw: selfExpr).\(raw: methodName)",
-                    returnType: returnType
-                )
-                append(item)
-            }
+        func callMethod(methodName: String, returnType: BridgeType) {
+            let (_, selfExpr) = removeFirstLiftedParameter()
+            generateParameterLifting()
+            let item = renderCallStatement(
+                callee: "\(raw: selfExpr).\(raw: methodName)",
+                returnType: returnType
+            )
+            append(item)
         }
 
         /// Generates intermediate variables for stack-using parameters if needed for LIFO compatibility
@@ -571,7 +561,7 @@ public class ExportSwift {
         if method.effects.isStatic {
             builder.call(name: "\(ownerTypeName).\(method.name)", returnType: method.returnType)
         } else {
-            builder.callMethod(methodName: method.name, returnType: method.returnType, isMutating: method.effects.isMutating)
+            builder.callMethod(methodName: method.name, returnType: method.returnType)
         }
         try builder.lowerReturnValue(returnType: method.returnType)
         return builder.render(abiName: method.abiName)

Plugins/BridgeJS/Sources/BridgeJSCore/SwiftToSkeleton.swift

@@ -1198,8 +1198,15 @@ private final class ExportSwiftAPICollector: SyntaxAnyVisitor {
             className: classNameForABI
         )
 
-        let isMutating = node.modifiers.contains { $0.name.tokenKind == .keyword(.mutating) }
-        guard let effects = collectEffects(signature: node.signature, isStatic: isStatic, isMutating: isMutating) else {
+        if let mutatingModifier = node.modifiers.first(where: { $0.name.tokenKind == .keyword(.mutating) }) {
+            diagnose(
+                node: mutatingModifier,
+                message: "@JS does not support mutating struct methods: mutations to 'self' cannot be propagated back to JavaScript",
+                hint: "Remove the mutating keyword or redesign the API to return the updated value instead"
+            )
+            return nil
+        }
+        guard let effects = collectEffects(signature: node.signature, isStatic: isStatic) else {
             return nil
         }
 
@@ -1214,7 +1221,7 @@ private final class ExportSwiftAPICollector: SyntaxAnyVisitor {
         )
     }
 
-    private func collectEffects(signature: FunctionSignatureSyntax, isStatic: Bool = false, isMutating: Bool = false) -> Effects? {
+    private func collectEffects(signature: FunctionSignatureSyntax, isStatic: Bool = false) -> Effects? {
         let isAsync = signature.effectSpecifiers?.asyncSpecifier != nil
         var isThrows = false
         if let throwsClause: ThrowsClauseSyntax = signature.effectSpecifiers?.throwsClause {
@@ -1235,7 +1242,7 @@ private final class ExportSwiftAPICollector: SyntaxAnyVisitor {
             }
             isThrows = true
         }
-        return Effects(isAsync: isAsync, isThrows: isThrows, isStatic: isStatic, isMutating: isMutating)
+        return Effects(isAsync: isAsync, isThrows: isThrows, isStatic: isStatic)
     }
 
     private func extractNamespace(