Keep author-layout image URLs editor-valid after WordPress materialization

## Problem

Fixture 37 materializes `blocks-engine/author-layout` with a nested image whose saved block markup retains the relative source `website/assets/materialized-svg/inline-svg-453b87c087581ebd.svg`. WordPress resolves the editor DOM value to an absolute theme URL, so `wp.blocks.validateBlock` reports an attribute mismatch and marks the block invalid.

## Evidence

- SSI PR: Automattic/static-site-importer#760 at `00cfb164`
- Blocks Engine: `74fec9e9`
- Homeboy run: `b280bf88-2257-4915-9d80-f4c52ff6a538`
- Fixture: `37-art-gallery-exhibition`
- Block: `blocks-engine/author-layout`
- Mismatch: relative `src=website/assets/materialized-svg/inline-svg-453b87c087581ebd.svg` vs absolute runtime theme URL

Reproduction:

```sh
node tools/run-fixture-matrix.mjs --static-site-importer /path/to/static-site-importer --blocks-engine /path/to/blocks-engine --local --target-fixture 37-art-gallery-exhibition --surface-coverage 6
```

## Acceptance

- Author-layout nested image serialization remains valid after WordPress resolves materialized asset URLs.
- The generic fix applies to URL-bearing nested content without fixture-specific selectors or paths.
- Fixture 37 editor validation reports no `author-layout` attribute mismatch.

## AI Assistance

OpenAI GPT-5.6 Sol via OpenCode ran the seven-surface matrix, analyzed the editor validation evidence, and drafted this report. Chris Huber reviewed the evidence and remains responsible for the issue.

Start by running the provided tools/run-fixture-matrix.mjs command against fixture 37-art-gallery-exhibition and inspect the blocks-engine/author-layout nested image serialization. Compare the relative materialized SVG source with the absolute URL WordPress uses during editor validation. Done means the generic URL-bearing nested-content handling passes validation without fixture-specific selectors or paths.

Written by the indexing model from the issue text.