fix(spec): keep project contract at v1

[vishr]

Summary

• restore the sole project contract identity to onebox.run/v1
• publish the current hostname-based schema at the stable v1 URL and remove v2 artifacts
• remove all legacy/transient snapshot migration and fallback code
• strictly validate predecessor snapshots before any deploy runtime mutation
• update docs, site examples, fixtures, tests, and native CI smoke coverage

Verification

• just ci
• focused app/engine tests: 1,367 passed
• Sol release-gate review: no major+ blockers