Rollup compute switch on the rows endpoint

## 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 (#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.

Start at the /cortext/v1/rows endpoint and trace how relation picker and command palette callers build their requests. Verify the current fields[] and rollup computation behavior first, then cover none, visible, and cached modes, including the cache-miss fallback and null rollup fields, with the relevant endpoint tests if present.

Written by the indexing model from the issue text.