fix(cli): avoid redundant snapshot cache rewrites

[garrettladley]

• rr --index-dir now tells users when it is loading or writing a ranked search snapshot so large caches no longer look like a frozen terminal
• clean snapshot loads skip the follow-up cache rewrite when there are no pending events.jsonl updates
• new snapshot.json writes use compact serde_json output instead of pretty-printed multi-line json
• cache size labels are formatted for the terminal status rows and pinned with a focused unit test