fix(cli): bound the loopback Studio probe so a dropped connection costs 1 s, not 10 - #6466

What

The loopback Studio probe in findPreviewServerForProject connects with node:net under a 1 s bound and destroys the socket before it fetches /api/projects. A host that drops the connection now costs 1 s instead of 10 s.

Why

Some hosts drop a connection to a closed loopback port instead of refusing it: WSL 2 with networkingMode=mirrored, or a firewall in stealth mode. On those hosts fetch("http://127.0.0.1:5190/api/projects") waits for undici's 10 s connect timeout. Every render, check and snapshot looks for a Studio through the history owner, so each of them paid that 10 s.

Adding an AbortSignal.timeout to the fetch is not enough. The fetch rejects on time, but the connect attempt keeps its socket open, and that keeps the process alive for the same 10 s. A net socket that is destroyed when its timeout fires lets the process exit at once.

To reproduce, on WSL 2 with mirrored networking and nothing on port 5190:

curl -m 15 http://127.0.0.1:5190/      # hangs 15 s; http://[::1]:5190/ is refused in 5 ms
node -e 'const t=Date.now();fetch("http://127.0.0.1:5190/",{signal:AbortSignal.timeout(1000)}).catch(()=>{});process.on("exit",()=>console.log(Date.now()-t))'   # prints ~10500

Related work

Also WSL: #5560, #5574, #5591 (renders fall back to software GPU under WSL). Fixes #6465.

How

• fetchLoopback(url): connects to the URL's host and port with node:net. It rejects and destroys the socket after 1 s, on an error, or once connected. After a successful connect it returns fetch(url).
• It is now the default fetchImpl of findPreviewServerForProject. The parameter type narrows to (url: string) => Promise<Response>, which is the only way it is called. Callers and tests that inject their own fetch are unchanged.

Test plan

• Unit tests added: fetchLoopback fetches from a listening loopback server, and rejects without fetching when the port refuses the connection. studioSelectionClient.test.ts: 15/15 pass.
• Manual testing on WSL 2 (mirrored networking, 12 cores, idle), hyperframes@0.8.205 with this change applied to its dist:
  • render of a 19.6 s composition: 27.8 s → 20.7 s
  • check: 13.0 s → 4.2 s (warm)
  • each command spent 10.5 s in fetch http://127.0.0.1:5190/api/projects, traced through a --require hook
• oxlint and oxfmt --check are clean on both files. tsc --noEmit in packages/cli is clean after bun run build.
• Two tests in history.test.ts ("a CLI run waits while another process holds the history…" and "serializes concurrent begin --exclusive claims…") time out at 20 s on this machine with and without this change. They are unrelated to it.
• Comments follow CONTRIBUTING.md "Comments".