Preserve native button inner-surface generated content

## Problem
Button conversion unwraps source inner-surface elements such as span.wsite-button-inner. Authored pseudo-element content and geometry attached to that surface disappear, removing arrows and shrinking native buttons.

## Direction
Project deterministic inner-surface generated content and geometry onto the native button link without retaining invalid nested button markup.

## Acceptance
- Weebly CTA arrows and source widths match.
- Native core/button markup remains editor-valid.
- Buttons without generated content do not regress.
- Transformer contracts and parity fixtures pass.

## AI assistance
OpenAI gpt-5.6-sol through OpenCode analyzed the parity evidence and drafted this issue. Chris Huber remains responsible for scope and implementation decisions.

Start with the transformer contracts and parity fixtures mentioned in the issue, then trace how Weebly CTA button inner-surface elements are converted. Compare generated content, geometry, and source widths against the fixtures. Done means arrows and widths match, core/button markup remains valid, buttons without generated content are unchanged, and the contracts and parity fixtures pass.

Written by the indexing model from the issue text.