fix: keep memory flat when precompressing many files

[Nic-Polumeyv]

builder.compress starts gzip and brotli streams for every file at once. Node runs zlib work on its threadpool (https://nodejs.org/api/zlib.html#threadpool-usage-and-performance-considerations), so the fan-out only costs memory: about 800 MB peak on 2000 files.

2000 files x 20 KB:

	time	peak memory
before	2.5 s	+800 MB
after	1.8 s	+14 MB

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 996da7e:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/996da7e3e03d3d3b7ec06bf3be90539864ebba13

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16993

[changeset-bot]

🦋 Changeset detected

Latest commit: 996da7e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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Preview: https://svelte-dev-git-preview-kit-16993-svelte.vercel.app/