Preserve CSS-significant inline and media geometry during block conversion

[chubes4]

Problem

The post-#623 15-saas browser matrix still fails exact visual parity despite producing 265/265 editor-valid native blocks and zero core/html blocks. The aligned mismatch improved from 36.74% to 24.89%, but the remaining drift is measurable transformer loss rather than WordPress outer-wrapper noise.

This blocks Automattic/static-site-importer#489.

Root causes

1. Selector-addressable inline <span> leaves are lowered to paragraph/mark structures. In the hero this changes an 18.14px inline label into a 28.05px paragraph and shifts the section by 9.91px. In .lm-meta, three independently styled pill spans collapse into one paragraph with mark children, losing sibling box geometry and author selector semantics.
2. Materialized SVG used dimensions do not preserve the source aspect ratio. At the same 574px width, the product SVG changes from 351.80px to 380px high.
3. CSS-significant structural group classes are dropped. The product right column loses its author identity and the grid row becomes 39.61px shorter, accumulating downstream position drift.
4. Lower impact: generated button wrappers are not layout-neutral when source styles target the anchor, changing the header CTA from 37.80px to 54.70px high.

Expected generic behavior

• Preserve selector-addressable inline semantic leaves, sibling boundaries, classes, and attributes when conversion to paragraph/mark changes authored CSS semantics.
• Preserve resolved SVG intrinsic/used aspect ratio in generated image block metadata and rendering.
• Preserve author classes on structural groups when referenced by authored CSS or required for descendant/child selector compatibility.
• Keep generated block wrappers layout-neutral when source geometry belongs to the represented leaf.

Acceptance criteria

1. Add deterministic transformer coverage for inline sibling pills, inline label line-height, SVG used dimensions, CSS-significant structural classes, and anchor-styled button wrappers.
2. Preserve native Gutenberg output and editor validity; do not introduce fixture-specific selectors or core/html fallbacks.
3. Rerun the single-fixture SSI matrix against 15-saas with exact visual gating.
4. Report source/candidate dimensions and mismatch ratio; promotion remains blocked unless exact 1:1 parity and human screenshot approval are achieved.

Reproduction

1. Check out Blocks Engine with Preserve resolved grid layout semantics #621 and Preserve author selector semantics across native block save shapes #623.
2. From a fresh Static Site Importer checkout, run its documented fixture-matrix wrapper against Blocks Engine with the authored saas tag and a Lab runner.
3. Inspect the source, candidate, diff, DOM snapshots, and visual attribution artifacts.

Supplementary Homeboy run: d2935000-19a7-4856-8703-5f9de249afb2. It recorded aligned mismatch 24.89%, raw mismatch 25.48%, no global offset, and 265/265 editor-valid native blocks.