Seed file stats from single-file checkpoints (classic and V2)

When seeding CRC context from a classic or V2 single-file checkpoint, we already scan the entire file to extract metadata fields. Since add/remove actions are in the same file, we should also extract file counts and sizes to produce `Valid` file stats instead of `Incomplete`.

This only applies to single-file checkpoints -- multi-part and V2 manifest+sidecar checkpoints have add/remove actions spread across multiple files, so the cost trade-off is different.

Follow-up to #1781.