Collapse typography-only text wrappers to styled paragraphs (#630)

[chubes4]

Summary

A styled <div>/<span> whose only content is inline text and whose CSS is typographic (font-size, letter-spacing, text-transform, color — no box-model geometry) was round-tripped as a core/group wrapping a default core/paragraph. That structure has two defects:

1. Font scale is lost — the wrapper class (e.g. .label{font-size:0.68rem}) applies to the group <div>, but the inner default paragraph renders at the theme's base paragraph size instead of inheriting the wrapper's scale.
2. Default block spacing is introduced — the inner paragraph gets a default block margin-top, adding vertical space the source never had.

On imported artist-site routes, an eyebrow like <div class="label">The Shop</div> rendered at the wrong size and injected a uniform ~21px top offset that shifted every following block down the page (measured: page-header inflated 458.6px → 479.7px; merch-intro/grid/footer all shifted +21px with no accumulation).

Fix

visualTextWrapperBlockFromElement() now emits a single styled core/paragraph carrying the wrapper class when the wrapper is a pure-text leaf (no child elements) with no box-model styling. Wrappers that own box-model geometry — padding, border, explicit sizing, or flex/grid layout (.tag, .tier-price, .use-case-result) — still round-trip as core/group so their block-level layout is preserved.

Box-model detection reads the raw matched declarations via structuralPresentationDeclarations() rather than the post-projection presentation set, so padding consumed into block-supports attributes is still seen.

Verification

Proven directly against the real fixture — fixtures/websites/3-artist-music/merch.html now compiles the eyebrow as:

<!-- wp:paragraph {"className":"label pl pl-d1","content":"The Shop"} --><p class="label pl pl-d1">The Shop</p><!-- /wp:paragraph -->

Full php-transformer suite green:

• 248 parity fixtures — no regressions
• 75 block-style-support conversion tests (box-model wrappers stay groups; typography-only .tier-name now collapses to a styled paragraph)
• wordpress-site-plan + shared-shell-plan contracts, HTML-to-blocks contract

The block-style-support tests were updated to encode the corrected contract: box-model wrappers stay core/group; typography-only text wrappers collapse to styled paragraphs.

AI assistance disclosure

Drafted with Claude (Sonnet 4.5) via opencode, used to root-cause the offset from DOM snapshots and implement the box-model-aware collapse. Every line reviewed by Chris Huber.

Refs #630.