fix(canonicalize): collapse arbitrary values into shorthand utilities (#19837)

The guard on `dynamicUtilities` restricted root-swapping to named
values, so arbitrary values like `px-[1.2rem] py-[1.2rem]` were never
collapsed into `p-[1.2rem]`.

This is what caused #19835 — in `--stream` mode, the collapse only
happened if an earlier line caused the shorthand to be registered in
`STATIC_UTILITIES_KEY` as a side effect, making the output
non-deterministic. The underlying issue is that arbitrary value collapse
wasn't supported at all.

The fix relaxes the guard from `parsedCandidate.value?.kind !== 'named'`
to `parsedCandidate.value === null`. `cloneCandidate` and
`printCandidate` already handle arbitrary values, so the root-swapping
machinery works without other changes.

The iteration in `dynamicUtilities` is over
`designSystem.utilities.keys('functional')` — a fixed set of roots, not
input-proportional — so the performance cost of including arbitrary
values should be negligible.

Fixes #19835.

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Co-authored-by: Robin Malfait <malfait.robin@gmail.com>