Blobs are typically hundreds of megabytes and should not be held in memory.
The new blobDiskCache writes cached blobs to a temp directory, tracks LRU
order in memory, and evicts least-recently-used files when total disk usage
exceeds a configurable limit (default 10 GiB).
Design:
- Blobs written to os.TempDir()/vaultik-blobcache-*/<hash>
- Doubly-linked list for O(1) LRU promotion/eviction
- ReadAt support for reading chunk slices without loading full blob
- Temp directory cleaned up on Close()
- Oversized entries (> maxBytes) silently skipped
Also adds blob_fetch_stub.go with stub implementations for
FetchAndDecryptBlob/FetchBlob to fix pre-existing compile errors.
