Project unmatched author p-selectors through source-tag marker; keep button rows out of navigation (#630)

[chubes4]

Summary

Two focused fidelity fixes for imported sites, both using existing BE mechanisms.

1. Project unmatched author type selectors through their source-tag marker

An author rule whose selector matched no source element — e.g. .page-header p in a fixture whose .page-header contains no source <p> (the intro paragraph lives elsewhere) — was emitted verbatim. Once a sibling <div> such as an eyebrow <div class="label"> collapses to a paragraph, that dormant .page-header p rule suddenly captures it at higher specificity and overrides its own class-owned type scale, inflating the element and shifting the whole page down.

BE already projects descendant p/li/nav type selectors through a source-tag marker carried only by elements that were that tag in the source (this is how .benefit-card p correctly avoids capturing a promoted .benefit-num eyebrow in the solved 15-saas fixture). That rewrite was simply skipped on the no-match branch, leaving the selector bare. This routes the no-match case through the same rewriteSourceTagTypes() path, so .page-header p becomes .page-header :where(.blocks-engine-source-p-…) and matches only real source paragraphs.

Note: an earlier revision of this PR added a parallel collapsed-paragraph exclusion mechanism. It duplicated the source-tag-marker system and regressed the already-solved 15-saas fixture (added markers, shifted the marker counter). That approach is removed. 15-saas's transform is now byte-identical to trunk, while the 3-artist-music eyebrow keeps its 0.68rem scale.

2. Keep button-styled link rows out of core/navigation

A row of button-styled links whose container carries an incidental navigational token (e.g. <div class="stream-links"><a class="stream-btn">…</a>…</div>, matched only because links looks navigational) was flattened into a core/navigation menu — halving the controls' height (39.7px pills → 20.5px text links). NavigationPattern now defers when a non-nav, non-list container's direct link children all carry button signals, so the row converts to core/buttons and keeps its pill geometry. Genuine nav menus, list navigation, and single incidental button-classed links are unaffected.

Verification

• 15-saas (solved fixture) transform is byte-identical to trunk — no regression.
• 3-artist-music: eyebrow keeps its type scale; streaming-links row restored to source parity.
• Full php-transformer suite green: 248 parity fixtures, 77 block-style-support tests, navigation + button unit suites, projection unit coverage, all contracts.

AI assistance disclosure

Drafted with Claude (Sonnet 4.5) via opencode, used to trace the cascade through in-sandbox CSS/DOM inspection, root-cause a self-introduced regression against the solved-fixture gate, and re-implement through the existing source-tag-marker mechanism. Every line reviewed by Chris Huber.

Refs #630.