fix(js): roll back module lifecycle initialization

README.md

@@ -339,6 +339,13 @@ comptime {
 }
 ```
 
+Module initialization is transactional:
+
+- If `init` fails, the environment refcount is restored and `cleanup` is not called.
+- If `init` succeeds but module registration fails, the environment refcount is restored and
+  `cleanup` is called.
+- After successful registration, `cleanup` runs when the environment exits.
+
 This enables safe shared-state initialization for worker thread scenarios.
 
 ---

src/js/export_module.zig

@@ -97,15 +97,27 @@ pub fn exportModule(comptime Module: type, comptime options: anytype) void {
             };
 
             var prev_refcount: u32 = 0;
+            var init_succeeded = false;
             if (has_lifecycle) {
                 prev_refcount = State.env_refcount.fetchAdd(1, .monotonic);
-                errdefer if (!cleanup_hook_registered) {
-                    _ = State.env_refcount.fetchSub(1, .acq_rel);
-                };
-
-                if (has_init) {
-                    try options.init(prev_refcount);
+            }
+            errdefer if (!cleanup_hook_registered) {
+                if (has_lifecycle) {
+                    const prev_refcount_rollback = State.env_refcount.fetchSub(1, .acq_rel);
+                    std.debug.assert(prev_refcount_rollback > 0);
+                    const new_refcount = prev_refcount_rollback - 1;
+
+                    if (has_cleanup) {
+                        if (init_succeeded) {
+                            options.cleanup(new_refcount);
+                        }
+                    }
                 }
+            };
+
+            if (has_init) {
+                try options.init(prev_refcount);
+                init_succeeded = true;
             }
 
             _ = try registerDecls(Module, env, module, 0);