page-status: BYOM content always shows 'No source'

For BYOM (Bring Your Own Markup) pages the `sourceLastModified` field from the admin API is `null`/absent. This causes `classifySequenceStatus` to always return **No source** (negative status indicator), even for pages that are fully previewed and published.

I can see a few options:
- somehow detect BYOM and treat explicitly differently (don't request edit in that case)
- don't quick exit when no source, proceed to rest of checks and label accordingly, something like `Current, No Source`
- some combination of the 2 above

Start by locating classifySequenceStatus and tracing how the admin API's sourceLastModified value is handled for BYOM pages. Check the existing page-status behavior for a fully previewed and published BYOM page; done should mean it is no longer always labeled No source, with the intended status behavior clarified.

Written by the indexing model from the issue text.