chore: tune vitest parallelism

vitest.e2e.config.ts

@@ -1,7 +1,14 @@
+import os from "node:os";
 import { defineConfig } from "vitest/config";
 
+const isCI = process.env.CI === "true" || process.env.GITHUB_ACTIONS === "true";
+const cpuCount = os.cpus().length;
+const e2eWorkers = isCI ? 2 : Math.min(4, Math.max(1, Math.floor(cpuCount * 0.25)));
+
 export default defineConfig({
   test: {
+    pool: "forks",
+    maxWorkers: e2eWorkers,
     include: ["test/**/*.e2e.test.ts"],
     setupFiles: ["test/setup.ts"],
     exclude: [