Add portable source manifest fixture

[chubes4]

Summary

• add the portable source manifest to solved fixture 89
• declare index.html and css/site.css as the complete website payload
• leave fixture.json available as corpus metadata without compiling it into generated themes

Dependency

This exercises the contract introduced by Automattic/static-site-importer#1416.

Verification

• validated the manifest JSON
• verified both declared SHA-256 hashes against the fixture bytes
• built an SSI development package from Add portable source manifest contract static-site-importer#1416
• imported this fixture through Add studio create --from for static sources studio#4725 using site create --from
• verified the generated theme excludes fixture.json and the portable manifest while retaining css/site.css

AI Assistance

OpenAI GPT-5.6-Sol via OpenCode was used to add the fixture manifest and run integrity and end-to-end verification. The change and evidence were reviewed and directed by Chris Huber.