Route merge-queue builds to self-hosted runner tiers with hosted fallback

### Task Summary

Merge-queue entries regularly wait on GitHub-hosted runner capacity: the ASF org shares one concurrency quota across all Apache projects, and the queue is the latency-critical serialized path. We now have 25 self-hosted runners registered for this repo (INFRA-provisioned), sized in four tiers (`arc-heavy-linux`, `arc-medium-linux`, `arc-light-linux`, `arc-bench-linux`). Add routing so merge-queue builds default to the self-hosted tiers while PR, post-merge, and scheduled runs stay on GitHub-hosted runners. A `ci:self-hosted` PR label opts a PR in for debugging. Routing must fall back to `ubuntu-latest` per tier when the fleet is offline or congested, so the pipeline never stalls on a runner outage.

### Task Type

- [x] DevOps / Deployment / CI