fix(router): honor SSR backpressure via drain event for compression compatibility

[maiieul]

What is it?

• Bug

Description

Since #8557 the Node SSR adapter resolves each chunk's write promise through res.write's per-chunk callback. compression (and similar wrappers) replace res.write with a 2-arg signature (chunk, encoding) and drop that callback, hanging streaming SSR mid-response.

Switch to Node's standard backpressure protocol — return value + 'drain' event — and resolve close() on 'finish', calling res.end() without a callback so compression@1.x's 2-arg res.end doesn't blow up on Buffer.from(<callback>).

	Pre-#8557	#8557	This PR
Pause when buffer full?	❌ never	✅ via per-write callback	✅ via 'drain' event
Robust to wrappers like compression?	(didn't matter — never paused)	❌ hangs if callback dropped	✅ — 'drain' / return value is the universal contract

Reproduced and verified end-to-end against a real Express + compression app.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5d2fe55

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LGTM! 🧑‍🌾