Keep imported inline SVG editor-valid with explicit sizing; convert SVG-in-link to core/button

[chubes4]

Problem

Imported inline <svg> elements fell back to core/html with no explicit sizing. When the source page sized them via stylesheet rules that are not applied to the imported candidate, they rendered at intrinsic/oversized dimensions — the dominant contributor to visual-parity mismatch on SSI fixture-matrix fixtures (up to 96% pixel mismatch on SVG-heavy pages).

Why not core/icon

An intermediate attempt routed inline SVG to core/icon. The package test suite passed — but the real editor invalidated 4/7 matrix fixtures: core/icon is a dynamic block whose registered contract carries a registered icon name (WP_Icons_Registry), not arbitrary SVG markup (verified against wordpress-develop: wp-includes/blocks/icon/block.json, icon.php, class-wp-icons-registry.php). Package-level validation cannot catch this class of error because CanonicalSaveShapeValidator intentionally skips dynamic blocks — only live editor validation surfaces it. That dead end is documented here so it isn't retried.

Fix

• Inline drawable SVG stays sanitized core/html, and viewBox-only SVGs get explicit width/height derived from the viewBox so rendering no longer depends on unapplied stylesheet rules.
• SVG inside button-like links is stripped from the label and converted to native core/button.
• External-sprite-only SVGs (<use href="…#id"> with no inlinable drawable content) remain a diagnostic fallback — the referenced symbol is outside the imported markup and would render broken.
• Parity fixtures added for decorative icon, sized hero graphic, and SVG-in-link shapes.

Verification

• Full suite green: composer test (canonical + 203 parity fixtures + packaging).
• SSI fixture matrix (complexity-1 lane, real WP sandbox): 7/7 pass, 0 editor-invalid blocks, aggregate native rate/block counts unchanged (the change lives inside fallback payloads by design). Pixel-level visual impact is deliberately not claimed here: the visual-parity channel currently swings too widely across runs to attribute (theme-stylesheet application on candidates is a known open issue tracked separately).

AI assistance

• AI assistance: Yes
• Tool(s): Claude Opus 4.5 (claude-fable-5) via opencode, orchestrated agent workflow
• Used for: Root-cause investigation, the core/icon contract verification against WordPress core source, implementation, and fixture-matrix verification. Reviewed and verified by Chris Huber via the SSI fixture matrix quality gates.