group-expand comparator plumbing: the setup/trigger machinery in bench-app is pretable-only

[blove]

Split out of #477, which made group comparative against TanStack. group-expand could be too — TanStack v9's free package ships everything needed (rowExpandingFeature, row.toggleExpanded()) — but the blocker is app machinery, not adapter capability:

• The pre-window setup in apps/bench/src/bench-app.tsx applies the grouping and then calls waitForGroupedRowModel(), which polls the pretable grid ref for the first group row.
• The measured trigger is grid.rowModel.setGroupExpanded(groupId, false) — a pretable engine call.

Making it comparative needs a comparator-generic version of both:

1. Setup: apply the plan override (already generic), then wait for grouped rendering via the DOM — count rows matching a per-profile group-row selector ([data-tanstack-group-row] exists as of feat(bench): make the group script comparative against TanStack #477; pretable's is [data-pretable-group-row]) instead of polling the engine.
2. Trigger: an adapter-exposed collapse handle, following the onUpdateApiReady pattern — e.g. onGroupToggleReady(collapseFirstGroup), where the tanstack implementation resolves the first group row in sorted order and calls row.toggleExpanded(false).
3. Plan arithmetic already holds: the post-collapse resultRowCount (leaves − collapsed group's members + all group rows) matches TanStack's expanded-flat-model semantics, per feat(bench): make the group script comparative against TanStack #477's finding that the two models agree on the expanded count exactly.

Worth carrying over from #477: the collapse must target the sorted-first group (the plan picks its probe row from the second group to dodge focus-reconciliation artifacts — see the comment on the group-expand plan in interaction-plan.ts), and the gate's rejection reason for tanstack should flip from "plumbing" to gone once this lands.

The grouped streaming scripts (group-updates, group-updates-stable-keys) are a separate, larger question — they need the comparator update path composed with grouping, and S5 local-max gates tuned for pretable's engine. Not covered by this issue.