feat: prepare 0.1.0-beta.5 candidate

[OpaceDigitalAgency]

What changed

• adds the composed JSON and Content Collection demo route
• adds the compact Simple/Complex demo switcher and shared-route source warning
• adds the reviewed two-file JSON/Markdown browser regression
• changes the public security contact to info@opace.co.uk
• documents the new public configuration without restoring internal planning files

Source-safety impact

• Owning file/path resolution: explicit source annotations and structured paths remain authoritative
• Ambiguity, stale-source and conflict behaviour: unchanged; all existing fail-closed checks remain
• Production-isolation impact: none; the editor remains development-only and the isolation gate passes
• Recovery or rollback impact: the multi-file browser workflow commits and restores both fixture sources

Validation

• Added or updated focused unit tests
• Added or updated Playwright coverage for user-visible behaviour
• Updated documentation and CHANGELOG.md when behaviour changed
• Ran npm run test:all
• Reviewed the packed package contents
• Reviewed the resulting Git diff

Local evidence: 29 unit tests and six Chromium workflows passed on candidate commit 76e7875.