Keep author-layout asset rewrites editor-valid

## Problem
The integrated Weebly parity candidate rewrites local asset URLs in saved `blocks-engine/author-layout` markup, but the block attribute still serializes the original relative content. Gutenberg compares the save output against persisted markup and reports five invalid blocks. The same fixture was fully editor-valid before the new bounded visual/image projection changes.

## Acceptance
- Asset publication/rewrite keeps `blocks-engine/author-layout` attributes and saved markup semantically identical.
- Local SVG and bounded visual assets render from published theme URLs.
- The four-surface Weebly fixture returns to 100% editor validity.
- Add deterministic contract coverage for relative-to-published asset rewriting inside author-layout content.

## Evidence
`/var/folders/lr/c_cmmt7s0592m4njz99v5yb40000gn/T/opencode/weebly-four-surface-direct-r59/summary.json` records five `editor_block_invalid` findings where relative `website/assets/materialized-svg/*.svg` attribute content differs from runtime-rewritten theme URLs.

## AI assistance
OpenAI gpt-5.6-sol through OpenCode analyzed r59 evidence and drafted this issue. Chris Huber remains responsible for scope and implementation decisions.

Start with the integrated Weebly parity candidate and the summary.json evidence, then trace publication and asset rewriting for saved blocks-engine/author-layout content. Add deterministic contract coverage for relative-to-published asset rewriting and verify that the four-surface fixture reaches 100% editor validity with local SVG and bounded visual assets rendering from published theme URLs.

Written by the indexing model from the issue text.