Rollup compute switch on the rows endpoint

[priethor]

Goal

Rollups are expensive, and some callers do not need them. Let callers skip rollups, compute only visible rollups, or use cached values once a cache exists.

Scope

• Add ?rollups=none|visible|cached to /cortext/v1/rows.
• none: skip compute_rollup_value. Requested rollup fields return null.
• visible default: compute only rollups included in the response. This pairs with fields[].
• cached: use cached values when present and fall back to compute. Activates once the rollup cache lands (Explore materializing collection field values and rollup cache for scale #177).
• Relation picker and command palette callers send ?rollups=none.

Acceptance criteria

• ?rollups=none skips rollup computation entirely, and rollup fields in the response return null.
• Default visible computes only rollups included in the response.
• cached falls back to compute when no cache exists.