Distribute EpisodeService across hosts #204

Context

EpisodeService holds running episodes in a local
dict[str, _RunningEpisode]. One process, one machine, one in-memory
registry. That's fine for the current use cases (eval scripts, single-
user demos).

Distributed training fleets need a different model:

• The trainer fans out N rollouts in parallel across M workers
• Each worker runs its episodes locally but the dispatcher needs to
  see global state (which episodes are live? which ones errored?)
• Snapshots and episode artifacts live on shared storage, not
  per-worker disks
• Health, restart, and stragglers need real handling

What this issue tracks

Generalize EpisodeService so it can run across multiple hosts.
Concretely:

• Pluggable backend for the in-memory registry (Redis / etcd /
  similar) — the dict becomes one impl, distributed becomes another
• Episode IDs that are globally unique (today they're per-process
  uuids — could collide across hosts in theory, ok in practice)
• Heartbeat / liveness so the orchestrator can detect a dead worker
• Shared storage abstraction for run artifacts (today's local
  filesystem becomes one impl)

Why it matters

This is the difference between OpenRange-as-tool (single-machine
eval, what we have) and OpenRange-as-platform (training fleets,
benchmark farms, multi-tenant). The architecture allows it; nothing
ships today that does it.

Where to start

• src/openrange/core/episode.py — the EpisodeService class.
  _RunningEpisode is the per-episode state today.
• src/openrange/core/store.py — the SnapshotStore is the
  filesystem-shaped artifact store.
• src/openrange/runtime.py — OpenRangeRun wraps the service +
  artifact store + dashboard. Distributed mode probably introduces
  a "controller" role on top of it.

Suggested phasing

Open a design doc PR first — distributed-systems shape decisions are
expensive to change later:

1. Define what "controller" / "worker" mean in the OpenRange model.
2. Pick the registry backend (Redis is the obvious low-effort
   choice).
3. Decide the shared-storage abstraction (S3 / NFS / pluggable).
4. Implement behind a flag.

Acceptance

• Design doc PR landed.
• A multi-host run works: 1 controller + N workers, each running
  episodes, all visible in one dashboard / report.
• Single-host mode still works unchanged (no perf regression for
  the common case).

Notes

Out of scope for this issue: actual training-loop orchestration on
top of distributed episodes — that's the trainer's concern (#198).
This issue is just the service primitive.

Pick this up when someone actually needs it. Filed for visibility.