refactor(eval): brainbench exits through the #2084 flushThenExit seam

The hand-rolled grace-tick predates the merge of the central write-fence +
aliveness-grace exit seam; same trap (PGLite exitCode hijack + Bun stdout
discard), now one tested implementation.

src/commands/eval-brainbench.ts

@@ -18,6 +18,7 @@
 import { readFileSync, writeFileSync, existsSync, mkdirSync } from 'node:fs';
 import { dirname, join } from 'node:path';
 import { execSync } from 'node:child_process';
+import { flushThenExit } from '../core/cli-force-exit.ts';
 import { cliOptsToProgressOptions, getCliOptions } from '../core/cli-options.ts';
 import { createProgress } from '../core/progress.ts';
 import { buildMetricGlossaryMeta } from '../core/eval/metric-glossary.ts';
@@ -216,14 +217,14 @@ function gitHeadSha(): string {
 
 /**
  * Bun discards queued stdout on process.exit and PGLite stomps
- * process.exitCode on its own ticks — hold the verdict locally, give the
- * event loop one grace window to drain pipes, then exit explicitly
- * (learnings pglite-emscripten-hijacks-process-exitcode +
- * bun-pipe-stdout-only-flushes-while-alive).
+ * process.exitCode on its own ticks — hold the verdict locally and exit
+ * through the #2084 central seam (write-fence + ref'd aliveness grace),
+ * which exists for exactly this trap.
  */
 async function exitWith(code: 0 | 1 | 2): Promise<never> {
-  await new Promise((resolve) => setTimeout(resolve, 300));
-  process.exit(code);
+  flushThenExit(code);
+  // flushThenExit exits after its bounded fence + grace; never resolve.
+  return new Promise<never>(() => {});
 }
 
 function readBaselineFile(path: string): BrainBenchBaseline {