Applying a theme migration

compareThemeLayouts() reports layout differences; applyThemeMigration() turns that report into a next draft only after every loss or addition has an explicit decision.

• Missing or incompatible current slots require discard before their content is omitted.
• Required new slots require add; optional new slots require either add or skip.
• Added sections are initialized from the component’s schema-approved default content.
• The returned result is a draft only. Authorization, review, and audit remain outside this helper. On Cloudflare, TenantWorkspaceThemeStore.switchDraft() atomically saves the returned document and target theme selection with optimistic versions; publishSite() later publishes that exact pair.

The initial implementation covers the current typed component registry. Future component mappings can add explicit conversion decisions rather than silently coercing incompatible content.