resourcemgr: demo, roachtest, dashboard

Goal: Build the debugging surface we need to validate and demonstrate resource-group-aware SQL CPU + admission behavior end-to-end.

Scope:
- Demo: a runnable script/cluster setup that drives two resource groups with different CPU weights and shows them being honored.
- Roachtest: an automated version of the same that asserts CPU share / latency / throughput differentials between groups.
- Dashboard: a Grafana / DB Console view that breaks down admission queue stats, CPU nanos, and slot usage by resource group.

Done when:
- Demo can be run locally with a single command and prints a clear pass/fail summary.
- Roachtest checked in and green nightly.
- Dashboard JSON committed under monitoring/ and renders per-group series in a smoke test cluster.

Epic: none

Jira issue: CRDB-64629