fix: serialize dynamic imports to prevent race condition on parallel account startup

Problem: When multiple Matrix accounts start in parallel, they all trigger
dynamic imports simultaneously. The Rust native crypto module
(@matrix-org/matrix-sdk-crypto-nodejs) crashes when loaded in parallel.

Solution: Add import-mutex.ts that caches import promises. Concurrent callers
now await the same promise instead of triggering parallel imports.

Files changed:
- NEW: src/matrix/import-mutex.ts - serializedImport() utility
- src/matrix/client/create-client.ts - use importCryptoNodejs()
- src/matrix/client/config.ts - use importCredentials()

src/matrix/client/config.ts

@@ -5,6 +5,7 @@ import type { CoreConfig, MatrixAccountConfig, MatrixConfig } from "../types.js"
 import { getMatrixRuntime } from "../../runtime.js";
 import { ensureMatrixSdkLoggingConfigured } from "./logging.js";
 import type { MatrixAuth, MatrixResolvedConfig } from "./types.js";
+import { importCredentials } from "../import-mutex.js";
 
 function clean(value?: string): string {
   return value?.trim() ?? "";
@@ -93,12 +94,13 @@ export async function resolveMatrixAuth(params?: {
   const isDefaultAccount = normalizedAccountId === DEFAULT_ACCOUNT_ID || normalizedAccountId === "default";
 
   // Only use cached credentials for default account
+  // Use serialized import to prevent race conditions during parallel account startup
   const {
     loadMatrixCredentials,
     saveMatrixCredentials,
     credentialsMatchConfig,
     touchMatrixCredentials,
-  } = await import("../credentials.js");
+  } = await importCredentials();
 
   const cached = isDefaultAccount ? loadMatrixCredentials(env) : null;
   const cachedCredentials =

src/matrix/client/create-client.ts

@@ -6,6 +6,8 @@ import {
   SimpleFsStorageProvider,
   RustSdkCryptoStorageProvider,
 } from "@vector-im/matrix-bot-sdk";
+
+import { importCryptoNodejs } from "../import-mutex.js";
 import type { IStorageProvider, ICryptoStorageProvider } from "@vector-im/matrix-bot-sdk";
 
 import { ensureMatrixSdkLoggingConfigured } from "./logging.js";
@@ -65,7 +67,8 @@ export async function createMatrixClient(params: {
     fs.mkdirSync(storagePaths.cryptoPath, { recursive: true });
 
     try {
-      const { StoreType } = await import("@matrix-org/matrix-sdk-crypto-nodejs");
+      // Use serialized import to prevent race conditions with native Rust module
+      const { StoreType } = await importCryptoNodejs();
       cryptoStorage = new RustSdkCryptoStorageProvider(
         storagePaths.cryptoPath,
         StoreType.Sqlite,

src/matrix/import-mutex.ts

@@ -0,0 +1,70 @@
+/**
+ * Import Mutex - Serializes dynamic imports to prevent race conditions
+ * 
+ * Problem: When multiple Matrix accounts start in parallel, they all call
+ * dynamic imports simultaneously. Native modules (like @matrix-org/matrix-sdk-crypto-nodejs)
+ * can crash when loaded in parallel from multiple promises.
+ * 
+ * Solution: Cache the import promise so that concurrent callers await the same promise
+ * instead of triggering parallel imports.
+ */
+
+// Cache for import promises - key is module specifier
+const importCache = new Map<string, Promise<unknown>>();
+
+/**
+ * Safely import a module with deduplication.
+ * If an import is already in progress, returns the existing promise.
+ * Once resolved, the result is cached for future calls.
+ */
+export async function serializedImport<T>(
+  moduleSpecifier: string,
+  importFn: () => Promise<T>
+): Promise<T> {
+  const existing = importCache.get(moduleSpecifier);
+  if (existing) {
+    return existing as Promise<T>;
+  }
+
+  const importPromise = importFn().catch((err) => {
+    // On failure, remove from cache to allow retry
+    importCache.delete(moduleSpecifier);
+    throw err;
+  });
+
+  importCache.set(moduleSpecifier, importPromise);
+  return importPromise;
+}
+
+// Pre-cached imports for critical modules
+let cryptoNodejsModule: typeof import("@matrix-org/matrix-sdk-crypto-nodejs") | null = null;
+let credentialsModule: typeof import("./credentials.js") | null = null;
+
+/**
+ * Safely import the crypto-nodejs module (Rust native).
+ * This is the most critical one - parallel imports of native modules crash.
+ */
+export async function importCryptoNodejs(): Promise<typeof import("@matrix-org/matrix-sdk-crypto-nodejs")> {
+  if (cryptoNodejsModule) return cryptoNodejsModule;
+  
+  const mod = await serializedImport(
+    "@matrix-org/matrix-sdk-crypto-nodejs",
+    () => import("@matrix-org/matrix-sdk-crypto-nodejs")
+  );
+  cryptoNodejsModule = mod;
+  return mod;
+}
+
+/**
+ * Safely import the credentials module.
+ */
+export async function importCredentials(): Promise<typeof import("./credentials.js")> {
+  if (credentialsModule) return credentialsModule;
+  
+  const mod = await serializedImport(
+    "../credentials.js",
+    () => import("./credentials.js")
+  );
+  credentialsModule = mod;
+  return mod;
+}