## Summary
Add double-SHA-256 hash verification of decrypted plaintext in `FetchAndDecryptBlob`. This ensures blob integrity during restore operations by comparing the computed hash against the expected blob hash before returning data to the caller.
The blob hash is `SHA256(SHA256(plaintext))` as produced by `blobgen.Writer.Sum256()`. Verification happens after decryption and decompression but before the data is used.
## Test
Added `blob_fetch_hash_test.go` with tests for:
- Correct hash passes verification
- Mismatched hash returns descriptive error
## make test output
```
golangci-lint run
0 issues.
ok git.eeqj.de/sneak/vaultik/internal/blob 4.563s
ok git.eeqj.de/sneak/vaultik/internal/blobgen 3.981s
ok git.eeqj.de/sneak/vaultik/internal/chunker 4.127s
ok git.eeqj.de/sneak/vaultik/internal/cli 1.499s
ok git.eeqj.de/sneak/vaultik/internal/config 1.905s
ok git.eeqj.de/sneak/vaultik/internal/crypto 0.519s
ok git.eeqj.de/sneak/vaultik/internal/database 4.590s
ok git.eeqj.de/sneak/vaultik/internal/globals 0.650s
ok git.eeqj.de/sneak/vaultik/internal/models 0.779s
ok git.eeqj.de/sneak/vaultik/internal/pidlock 2.945s
ok git.eeqj.de/sneak/vaultik/internal/s3 3.286s
ok git.eeqj.de/sneak/vaultik/internal/snapshot 3.979s
ok git.eeqj.de/sneak/vaultik/internal/vaultik 4.418s
```
All tests pass, 0 lint issues.
Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #39
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
Co-committed-by: clawbot <sneak+clawbot@sneak.cloud>
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.
