chore: add a read-only script to compare execution snapshots across stores

Adds `scripts/compare-snapshot-stores.mts`, a read-only operator script
that checks the Redis execution snapshot store against Postgres. For
every run in a time window whose first snapshot is marked mirrored, it
compares the snapshot ids, every entry field, the valid-entry index and
the head, and writes any differences as JSON lines.

It only issues SELECTs against Postgres and HGETALL, ZRANGE, GET and
EXISTS against Redis, so both can point at read replicas. `--check`
tests the connections without comparing anything, and `--limit N` stops
after N runs.

Mono-RevId: acd8711dab062906530b9b144c60a3f3b2e3b4a1

Author: d-cs