Pile Blob Metadata

The pile file already stores a timestamp and length in each blob header but this information is discarded when building the in-memory index. Clients therefore cannot query when a blob was added or how large it is without re-parsing the file.

Proposed Changes

• Extend IndexEntry in src/repo/pile.rs with a timestamp field. The length can be determined from the stored bytes when needed.
• Introduce a public BlobMetadata struct containing timestamp and length so callers do not depend on internal types.
• Populate the timestamp when Pile::try_open scans existing entries and when inserting new blobs. Lengths are computed on demand.
• Add PileReader::metadata(&self, handle) to retrieve a blob's metadata if it exists. Iterators may later be extended to yield this information alongside the blob itself.

This approach keeps the current API intact while making useful details available for replication and debugging tools.