fix(cli): skip oversized snapshot cache loads

[garrettladley]

• rr --index-dir now treats oversized snapshot.json files as bad warm-cache candidates instead of spending minutes parsing them
• giant snapshots rebuild through the normal indexed corpus path and write the compact cache format from main
• normal-sized snapshots still load directly and keep the clean-load no-rewrite behavior from the previous cache fix
• the snapshot load budget is pinned with a focused unit test