A Layout with no display rules previously showed every schema property under
"Hidden · N" with eye-off icons while also stating "All properties shown in
schema order". An empty allowlist means "show all", but the editor rendered the
empty case as "nothing shown", so the editor contradicted the actual rendering
and the first "Show" click left only a single property visible.

Replace the two-section Shown/Hidden editor with a single reorderable list of
all schema properties:

- Each row has a visibility toggle. The eye / eye-off icon reflects the current
  state (matching Coda, Notion, Airtable); a tooltip names the action.
- Hidden rows dim and sink to the bottom and are not draggable (only shown rows
  carry a drag handle).
- The header shows "N of M shown".
- "Show all properties" appears only when a property is hidden, and reveals the
  hidden properties while preserving the existing order.
- The last visible property's hide toggle is disabled (tooltip: "At least one
  property must be shown"), so the list can never collapse to zero shown and
  flip back to showing everything. A true "show none" state is left for later
  (a non-breaking add, since "show all" is persisted as the absence of the key).

Domain semantics are unchanged: an empty display-rule list still means "show all
properties in schema order". Display-rule editing logic is extracted into a pure,
unit-tested module.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

… drop guard

Address review findings on the unified show/hide list:

- The last shown property's hide toggle is disabled, but its explanatory
  title sat on the disabled button, where browsers suppress the native
  tooltip. Move the title onto a wrapping span (with pointer-events: none on
  the disabled button) so the "at least one property must be shown"
  explanation stays reachable on hover; the aria-label stays on the button.
- Hidden rows shared the single sortable list as drop targets, so a shown row
  could be dropped into the hidden region and snap back on re-render. Add a
  draggable passthrough to useSortable and use it to exclude hidden rows from
  sorting.
- Unify the test fixtures on newTextProperty and a shared newDisplayRules
  helper, dropping the duplicated local rules() builders.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>