Move CSS custom-property resolution onto StyleResolver (#242)

[chubes4]

Part of #242, workstream 1. Prerequisite for the SvgMaterializationTrait extraction.

resolveCssVariablesInValue() expands var(--token) references against source custom properties. It lived in SvgMaterializationTrait.

The threading was the tell

Before this change, resolving a CSS variable took this route:

1. HtmlTransformer passes a closure into StyleResolutionContext, wrapping a method it inherits from SvgMaterializationTrait.
2. StyleResolver calls back out through that closure.
3. The method — defined in the SVG trait — calls $this->styleResolver->structuralPresentationDeclarations().

So a CSS concern was defined in the SVG mixin, threaded through a context, and called back into the very collaborator it needed. Its two dependencies were sourceStyles() and a StyleResolver method: neither has anything to do with SVG.

I found this while measuring SvgMaterializationTrait for extraction. It is worth landing on its own rather than inside that slice, because it changes call sites across four files and is easier to review as one idea.

What changed

resolveCssVariablesInValue() becomes a public method on StyleResolver. Both dependencies resolve locally — sourceStyles() through the resolver's own context, structuralPresentationDeclarations() directly — so the closure leaves StyleResolutionContext entirely.

	before	after
StyleResolutionContext operations	15	14
SvgMaterializationTrait lines	983	944
SvgMaterializationTrait external deps	20	19

Call sites now reach it directly on the resolver: 16 in HtmlTransformer, 2 in SvgMaterializationTrait, and 3 inside StyleResolver that previously bounced through the context to reach code that called back into StyleResolver.

A check that was wrong, and how it surfaced

After rewiring HtmlTransformer I grepped for stale references with a filter that excluded $this->resolveCssVariablesInValue( — which was exactly the broken pattern. It reported clean.

The first fixture transform then failed immediately:

Call to undefined method HtmlTransformer::resolveCssVariablesInValue()
  in SvgMaterializationTrait.php:485

Two call sites inside the SVG trait itself, plus StyleResolver's three, were still unrouted. Fixed, then re-checked across the whole src tree rather than one file.

Noting it because the pattern keeps recurring in this workstream: a verification step that excludes the failure mode it is meant to detect. Same shape as the vacuous markup hash corrected on #1333.

Verification

serialized_blocks SHA-256 plus block, diagnostic, fallback, asset and coverage fingerprints, captured on trunk at fcca8efa and recaptured here:

383 documents — DIFFERING: 0 — threw: 0
markup: 22,053,531 bytes across 383 distinct hashes

composer test exit 0.

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AI assistance disclosure: implemented and drafted by Claude Sonnet 4.6 running in Claude Code, operated by @chubes4. The AI found the misplacement while measuring SvgMaterializationTrait for a separate extraction slice, performed the move, diagnosed the undefined-method failure caused by its own faulty grep filter, and captured the before/after corpus comparison quoted above. Reviewed by a human before opening.