fix: also serialize matrix/index.js import in channel.ts

The parallel account startup was crashing because the dynamic import
of ./matrix/index.js returned undefined when called concurrently.

Added importMatrixIndex() to import-mutex.ts and use it in channel.ts
to ensure the module is only imported once during parallel startup.

src/channel.ts

@@ -21,6 +21,7 @@ import {
   type ResolvedMatrixAccount,
 } from "./matrix/accounts.js";
 import { resolveMatrixAuth } from "./matrix/client.js";
+import { importMatrixIndex } from "./matrix/import-mutex.js";
 import { normalizeAllowListLower } from "./matrix/monitor/allowlist.js";
 import { probeMatrix } from "./matrix/probe.js";
 import { sendMessageMatrix } from "./matrix/send.js";
@@ -403,7 +404,8 @@ export const matrixPlugin: ChannelPlugin<ResolvedMatrixAccount> = {
         `[${account.accountId}] starting provider (${account.homeserver ?? "matrix"})`,
       );
       // Lazy import: the monitor pulls the reply pipeline; avoid ESM init cycles.
-      const { monitorMatrixProvider } = await import("./matrix/index.js");
+      // Use serialized import to prevent race conditions during parallel account startup.
+      const { monitorMatrixProvider } = await importMatrixIndex();
       return monitorMatrixProvider({
         runtime: ctx.runtime,
         abortSignal: ctx.abortSignal,

src/matrix/import-mutex.ts

@@ -68,3 +68,21 @@ export async function importCredentials(): Promise<typeof import("./credentials.
   credentialsModule = mod;
   return mod;
 }
+
+// Pre-cached import for matrix index module
+let matrixIndexModule: typeof import("./index.js") | null = null;
+
+/**
+ * Safely import the matrix/index.js module.
+ * This is called from channel.ts during parallel account startup.
+ */
+export async function importMatrixIndex(): Promise<typeof import("./index.js")> {
+  if (matrixIndexModule) return matrixIndexModule;
+  
+  const mod = await serializedImport(
+    "./matrix/index.js",
+    () => import("./index.js")
+  );
+  matrixIndexModule = mod;
+  return mod;
+}