Convert static UI fragments without fallback blocks and preserve layout geometry

[chubes4]

Problem

The SSI production gate requires exact visual parity and zero fallback blocks. A fresh 87-fixture matrix run against current Blocks Engine trunk found a representative static site that imports successfully but still emits fallback block families for:

• a search input wrapper
• a static empty-state/editor canvas shell
• a decorative color swatch
• a progressively enhanced email form

The same output renders 451 px taller than the source and has a 70.90% overlap pixel mismatch. Capture readiness was high on both sides; all stylesheets and images loaded. The mismatch is therefore transformed structure/style geometry, not an incomplete browser capture.

Expected behavior

• Generic static wrappers and controls map to editable native blocks whenever their preserved semantics can be represented by existing WordPress blocks.
• Decorative data/style spans map without core/html.
• Forms use the available native/provider form block contract without being reported as fallback HTML.
• Generated grouping, spacing, typography, and dimensions preserve source geometry closely enough to pass exact visual parity.
• Recognizers remain generic and are covered by representative DOM-shape tests, with no fixture-specific selectors or names.

Acceptance criteria

• Zero core/html or fallback findings for the four static fragment families above.
• Editor validation remains fully valid.
• The representative static site no longer has a dimension mismatch and reaches exact visual parity.
• Existing generic transformer tests and the targeted fixture matrix lane pass.

Relates to Automattic/static-site-importer#489 and Automattic/static-site-importer#651.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode with OpenAI GPT-5.6 Sol
• Used for: Ran the matrix, analyzed typed fallback and visual attribution evidence, and drafted the tracker. Chris reviewed and owns the issue.