fix: don't send Vary: Accept-Encoding for assets that were never precompressed

[Nic-Polumeyv]

The remaining half of #10343, and the first concrete piece of #16565.

#15182 stopped the header when precompress is false. It still goes out on every asset when precompress is true, because sirv sets Vary from its options rather than from the file, while builder.compress only compresses a fixed extension list. So a .ico or a .png advertises a variant that was never produced.

Only extensions that were written but never compressed lose the header, since setHeaders sees the request pathname, not the file sirv resolved, and / or /v1.0 may be serving index.html.gz.

Commits are separated by layer so the kit-side and adapter-side changes can be reviewed apart.

This rewrites the same setHeaders block as #16564, so whichever lands first, the other needs a rebase.

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from afccee5:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/afccee5e3bfcc6ca44185e13d989ac756367a911

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16566

Note

This PR is from a fork. A maintainer must approve approve each commit before it can be built and installed.

[changeset-bot]

🦋 Changeset detected

Latest commit: afccee5

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

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/kit	Minor
@sveltejs/adapter-node	Patch

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