1.0.0-alpha6: operator-UX polish from alpha5 field testing

Three operator-UX nits surfaced by alpha5 field testing on an
LSCache-fronted Drupal 11 site. None changed alpha5 correctness,
but together they made the lead diagnostic look broken on edge-
cached stacks. No API changes.

- Cache state comparison falls back from drupal_cache.page to
  drupal_cache.dynamic when page is empty.
- Empty-state copy mentions both cwv-pc and cwv-dc.
- README documents cwv-pc non-availability on edge-cached stacks.
- README's developer-API column list corrected.

See ROADMAP.md for full details.