Follow-up to #16374. Wanted to do this for a long time. Slightly
experimental — anything sourcemap-related always needs battle-testing —
but I'm reasonably sure it won't do any harm in cases where it doesn't
work perfectly.

Whereas #16374 made sourcemaps work during prerendering, this makes them
work in production generally, which includes prerendering and `vite
preview`. It applies three changes:

- it traces locations back to the source on a best-effort basis
- it makes paths relative to the cwd — much shorter and more readable,
but still cmd-clickable
- it truncates the framework's internal stack frames from the bottom

Additionally, it configures Vite to generate sourcemaps for the server
build (but allows that config to be overwritten, if someone really
doesn't want sourcemaps for whatever reason).

This is `vite preview` on `version-3`...

<img width="1379" height="260" alt="image"
src="https://github.com/user-attachments/assets/ebdab846-2beb-4e3b-83e2-0f3527078055"
/>

...and this is on this branch:

<img width="577" height="116" alt="image"
src="https://github.com/user-attachments/assets/d4e4810b-5cd4-4044-bf35-4a3f33d7d38c"
/>

It's not bulletproof. Taking `adapter-node` as an example, if the
emitted `build` directory was deployed without the `.svelte-kit`
directory alongside it, it wouldn't be able to trace all the way back to
the source. For that to happen, we would need to do something like add a
method on `builder` that bundled the sourcemaps, or moved (and adjusted)
them to a new location that _could_ be shipped with the build.

Similarly if an adapter moves files around, or does an additional
bundling step _without_ generating sourcemaps, there's not much we can
do.

But these are fixable things that we can address once this layer is in
place, and we shouldn't let perfect be the enemy of the good.

---

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---------

Co-authored-by: Tee Ming <[email protected]>

tiny fix which should have been part of #16374 — we don't need to log
responses multiple times. will self-merge once green

This PR was opened by the [Changesets
release](https://github.com/changesets/action) GitHub action. When
you're ready to do a release, you can merge this and the packages will
be published to npm automatically. If you're not ready to do a release
yet, that's fine, whenever you add more changesets to version-3, this PR
will be updated.

⚠️⚠️⚠️⚠️⚠️⚠️

`version-3` is currently in **pre mode** so this branch has prereleases
rather than normal releases. If you want to exit prereleases, run
`changeset pre exit` on `version-3`.

⚠️⚠️⚠️⚠️⚠️⚠️

# Releases
## @sveltejs/[email protected]

### Major Changes

- breaking: `config` exported from a universal route file takes
precedence over a server one
([#16400](#16400))

- breaking: consistent special filename patterns
([#16382](#16382))

### Minor Changes

- feat: support sourcemaps in production
([#16412](#16412))

- feat: support function validators for environment variables
([#16402](#16402))

- feat: better error logging
([#16374](#16374))

### Patch Changes

- fix: don't treat callable standard schemas as function param matchers
([#16403](#16403))

- fix: reject malformed streamed data encoding
([#16423](#16423))

- fix: hide stack traces for internal errors like 404s
([#16411](#16411))

- perf: match only unpaired surrogates when escaping HTML
([#16407](#16407))

- fix: don't report empty environment variables as missing
([#16401](#16401))

- chore: clarify which hooks run during server route resolution
([#16397](#16397))
## @sveltejs/[email protected]

### Minor Changes

- feat: better error logging
([#16374](#16374))

### Patch Changes

- Updated dependencies
[[`5220191`](5220191),
[`8cb2f7d`](8cb2f7d),
[`b88c7a7`](b88c7a7),
[`a6ea113`](a6ea113),
[`6446f64`](6446f64),
[`58f1789`](58f1789),
[`09774a2`](09774a2),
[`c542fdd`](c542fdd),
[`aedaa27`](aedaa27),
[`428ee1a`](428ee1a),
[`fefb3ae`](fefb3ae)]:
  - @sveltejs/[email protected]

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

#16374 (comment)