resourcemgr: simulation testing for resource groups

**Describe the problem**

Goal: Use the existing admission control simulator (pkg/util/admission/...sim...) to model resource-group behavior under controlled load so we can reason about fairness/starvation without spinning up a cluster every time.

Scope:
- Extend the simulator (or add a sibling) to support multiple resource groups with different weights and arrival patterns.
- Capture scenarios: equal weights, skewed weights, one group bursty / one steady, low-priority group competing with high-priority group, group with no work.
- Output per-group throughput, queue depth, and latency.

Done when:
- Simulator scenarios committed as tests with golden outputs.
- At least one scenario documents a known behavior we previously could only observe in roachtests.

Epic: none

Jira issue: CRDB-64630