│ feat(res): add res.sse() for Server-Sent Events

[danelmott]

Summary

Adds a res.sse() helper for Server-Sent Events, so apps no longer hand-write the
text/event-stream headers, the wire format (data:/event:/id:/retry:), a
keep-alive heartbeat and the disconnect cleanup — the same way res.json() wraps
JSON.stringify + headers.

app.get('/events', (req, res) => {
  const sse = res.sse()
  sse.send({ hello: 'world' })                 // object -> JSON
  sse.send('tick', { event: 'ping', id: 1 })   // named event
})

The client uses the standard EventSource API:

js
const es = new EventSource('/events')
es.onmessage = (e) => console.log(e.data)
es.addEventListener('ping', (e) => console.log(e.data))

API

res.sse([options]) opens the stream and returns an SSE instance:

- send(data, [options]) — send one event (chainable). options: event, id, retry.
- stream(source, [options]) — emit one event per chunk; resolves when the source ends.
- comment([text]) — keep-alive comment, ignored by the client (chainable).
- close() — stop the heartbeat and end the response (chainable).

send() serializes data consistently with the rest of the framework:

┌─────────────────────┬─────────────────────────┐
│        data         │         Sent as         │
├─────────────────────┼─────────────────────────┤
│ string              │ as-is                   │
├─────────────────────┼─────────────────────────┤
│ object / array      │ JSON.stringify          │
├─────────────────────┼─────────────────────────┤
│ number / boolean    │ String(data)            │
├─────────────────────┼─────────────────────────┤
│ Buffer / TypedArray │ UTF-8 text              │
├─────────────────────┼─────────────────────────┤
│ multi-line          │ one data: line per line │
└─────────────────────┴─────────────────────────┘

stream() accepts any web ReadableStream (e.g. a fetch() body), Node Readable,
Blob, or async iterable.

Heartbeat

A keep-alive comment is sent every 15s by default so proxies don't drop idle
connections. Configurable or disabled per stream: res.sse({ heartbeat: 10000 }) /
res.sse({ heartbeat: false }). The timer is unref()'d and cleared automatically on
disconnect via on-finished.

Design notes

- No new dependencies — reuses debug/on-finished and built-in Node APIs.
stream() unifies sources into a Node Readable via Readable.fromWeb().
- New module lib/sse.js (kept separate like lib/view.js); res.sse is a thin
method in lib/response.js.
- Broadcasting to multiple clients is intentionally left out to keep this PR small.

Tests

test/res.sse.js (mocha + supertest): headers, all data types, event/id/retry,
multi-line, comments, chaining, write-after-close, and stream() with a web
ReadableStream, Node Readable, async iterable and Blob. Full suite passes with
--check-leaks; npm run lint is clean.

Refs #7334