Performance: Request only visible row fields

[priethor]

What

Closes #172.

When a table view hides custom columns, row requests now send a fields[] list so the API only formats the fields the view needs. Full/default views still fetch the full row, and client-mode views keep the full payload because local filters and calculations may need hidden fields.

Why

Hidden rollups and relations can still be expensive to format. Skipping them keeps table loads cheaper without changing what the user sees.

How

• The rows endpoint accepts fields[] and ignores stale or system keys.
• The editor sends a sorted field list only when it is a real subset.
• The perf bench now measures full heavy-rollup rows separately from a rollup-only projection.

Testing Instructions

1. Open a collection table with several custom fields, including a relation or rollup.
2. Hide one of the custom fields and reload the table.
3. Confirm the visible cells still render correctly.
4. Reorder visible columns and confirm the table does not refetch just because the order changed.
5. Show all custom fields again and confirm the table loads normally.

Screen recording

Grabacion.de.pantalla.2026-05-22.a.las.16.57.29.mov