perf: default extension vitest lanes to threads

vitest.extensions.config.ts

@@ -23,6 +23,7 @@ export function loadIncludePatternsFromEnv(
 export default createScopedVitestConfig(
   loadIncludePatternsFromEnv() ?? ["extensions/**/*.test.ts"],
   {
+    pool: "threads",
     // Channel implementations live under extensions/ but are tested by
     // vitest.channels.config.ts (pnpm test:channels) which provides
     // the heavier mock scaffolding they need.

vitest.scoped-config.ts

@@ -1,9 +1,14 @@
 import { defineConfig } from "vitest/config";
 import baseConfig from "./vitest.config.ts";
 
-export function createScopedVitestConfig(include: string[], options?: { exclude?: string[] }) {
+export function createScopedVitestConfig(
+  include: string[],
+  options?: { exclude?: string[]; pool?: "threads" | "forks" | "vmForks" },
+) {
   const base = baseConfig as unknown as Record<string, unknown>;
-  const baseTest = (baseConfig as { test?: { exclude?: string[] } }).test ?? {};
+  const baseTest =
+    (baseConfig as { test?: { exclude?: string[]; pool?: "threads" | "forks" | "vmForks" } })
+      .test ?? {};
   const exclude = [...(baseTest.exclude ?? []), ...(options?.exclude ?? [])];
 
   return defineConfig({
@@ -12,6 +17,7 @@ export function createScopedVitestConfig(include: string[], options?: { exclude?
       ...baseTest,
       include,
       exclude,
+      ...(options?.pool ? { pool: options.pool } : {}),
     },
   });
 }