feat(ci): xdist sanity check workflow + docs

- backend-xdist.yml: copy of backend.yml with xdist flags, per-worker
  Snuba bootstrap, PYTHONHASHSEED=0, skip selenium plugin, fast-path
  calculate-shards
- tiered-xdist-changes.md: detailed changelog for all changes
- notes.md: design decisions and investigation notes
- memory.md: comprehensive reference for future work

Author: mchen-sentry