## Summary

Move scheduler Miri tests from 16 vCPUs / 32 GiB to 4 vCPUs / 8 GiB, and
migration checks from 8 vCPUs / 16 GiB to 4 vCPUs / 8 GiB. Reuse the
existing `LINUX_MEDIUM` profile and keep test commands and coverage
unchanged.

September 16 whole-runner CPU samples showed Miri averaging about 1.3
cores with a 2.4-core sampled peak and 2.6 GiB peak RAM; migration
checks averaged 1.2 cores with a 1.8-core sampled peak and 1.3 GiB peak
RAM. These are sampled observations, not a cold-build guarantee.

## Validation

- `cargo xtask workflows`
- `cargo fmt -p xtask -- --check`
- `git diff --check`
- CI on the smaller runners is pending.

## Human review

The root repository rules require the README review notice. The human
author must remove it after review; this PR remains a draft until that
step and CI verification are complete.

Release Notes:

- N/A