test(runtime-tests): run PTY tests concurrently by default

With resize rendering now synchronous, no PTY test depends on wall-clock
timing, so concurrent execution is safe. Enable sequence.concurrent: true.
Each test already spawns its own isolated PTY subprocess (or in-process app)
with no shared state.

Verified stable: 12/12 runs green under sequence.concurrent with forks capped
to 4 (mimicking a 4-core CI runner) — the configuration that reliably flaked
before the resize fix. No it.sequential opt-outs are needed.

packages/runtime-tests/vitest.pty.config.ts

@@ -13,6 +13,11 @@ export default defineConfig({
     // test) to absorb CPU contention on smaller CI runners.
     pool: "forks",
     fileParallelism: true,
+    // Tests run concurrently by default. Each spawns its own isolated PTY
+    // subprocess (or in-process app) with no shared state, and rendering is no
+    // longer wall-clock-dependent (resize renders synchronously), so exact
+    // render-count assertions stay deterministic even under CPU contention.
+    sequence: { concurrent: true },
     testTimeout: 15000,
     env: { FORCE_COLOR: "3" },
   },