Adds `RedisSnapshotStore` to `@internal/run-store`: a Redis-backed,
append-only store for a run's execution-state log, as an alternative to
keeping that log in Postgres.
Nothing constructs it. No existing code path can reach it, so merging
this changes no behaviour. The store, the wiring that would use it, and
the switch that would enable it are deliberately separate changes.
## Design
Four keys per run, plus one key per wait cycle, all sharing a `{runId}`
hash tag. Every mutation for a run therefore lands in one cluster slot,
and each operation is a single Lua script.
No script mints a key name. Dynamic keys are derived from `KEYS[1]` by
string surgery, because ioredis applies `keyPrefix` only to the KEYS
array: a key built inside Lua would be unprefixed while the client wrote
a prefixed one.
Retention is keyed to run completion. A non-terminal run's keys carry no
expiry at all, since a suspended run can wait indefinitely with nothing
left to refresh a TTL. The terminal transition sets the completion
expiry once, and a write arriving after completion re-applies that same
expiry rather than a live one, so a stale client cannot resurrect a key.
Entry JSON round-trips byte for byte. No script calls `cjson`, and the
values the store assigns itself live in their own hash fields instead of
being patched into the caller's document.
Sizes are observed, never enforced. Entry and cycle-key bytes are
recorded, with a warning above a configurable mark. Nothing rejects,
truncates, or spills.
`append` takes an optional expected-current-snapshot argument. Left out,
it advances the pointer unconditionally, matching the Postgres behaviour
it replaces. Supplied, it advances only on a match and otherwise reports
the conflict without writing.
Covered by 48 tests against a real Redis container, including the
retention transitions, the single-slot guarantee under a key prefix, and
tenant-scoped reads.
Daniel Sutton
dd3a1c0c54