Project columns in the rows endpoint

[priethor]

Goal

The rows endpoint formats every field on every row, even when a view only shows a subset. Let callers ask for the fields they need so the server work tracks what gets rendered.

Scope

• Add ?fields[]=title&fields[]=field-123 to /cortext/v1/rows. If omitted, keep formatting everything.
• format_row honors the requested list. System fields (title, created_at, modified_at, created_by, modified_by) are always included.
• When a rollup field is requested, resolve its source field server-side without returning that source field.
• The DataView block sends its visible columns and the dependencies needed by visible rollups.

Depends on

• Reproducible bench harness for collection performance #171

Acceptance criteria

• A response with fields[] returns only those fields plus the always-included system fields.
• The rollup-heavy benchmark gets noticeably faster, targeting about 3x for a 10-of-30 view.
• Callers that omit fields[] keep working unchanged.