Validate blob hashes, offsets and lengths from the destination (closes #155)

A blob hash read back from the downloaded snapshot database or the store listing was trusted unchecked. A hostile remote could set a hash such as "aa/../../etc" and have a decrypted blob written outside the cache directory, or feed a short or negative value that panicked a command.

blobDiskCache.path now refuses any key with a path separator, and ReadAt rejects a negative offset or length, bounding so a sum cannot overflow past the check. A new isBlobHash helper gates FetchBlob, shallow and deep verify, and restore: buildBlobIndexes rejects every hash from the snapshot database before any fetch. The blobs/ and metadata/ listings skip a non-conforming name, and short-hash prefixes in log and error text go through a panic-safe shortHash helper.

Model: opus-4-8

internal/vaultik/restore.go

@@ -425,12 +425,12 @@ func (s *restoreSession) downloadNextBlobSet(plan *restorePlan) (bool, error) {
 
 		blob, ok := s.blobByHash[hash]
 		if !ok {
-			return false, fmt.Errorf("%w: %s", errBlobMissingFromIndex, hash[:16])
+			return false, fmt.Errorf("%w: %s", errBlobMissingFromIndex, shortHash(hash))
 		}
 
 		err := s.downloadBlobToCache(hash, blob.CompressedSize)
 		if err != nil {
-			return false, fmt.Errorf("downloading blob %s: %w", hash[:16], err)
+			return false, fmt.Errorf("downloading blob %s: %w", shortHash(hash), err)
 		}
 
 		s.result.BlobsDownloaded++
@@ -474,6 +474,16 @@ func (v *Vaultik) buildBlobIndexes(
 	blobByHash := make(map[string]*database.Blob, len(blobsByID))
 	for id, blob := range blobsByID {
 		hash := blob.Hash.String()
+
+		// The snapshot database is untrusted. A hash that is not 64
+		// lowercase hex characters could steer a later fetch to a path
+		// outside the cache directory, so reject it here, before any
+		// blob is downloaded.
+		if !isBlobHash(hash) {
+			return nil, nil, fmt.Errorf(
+				"%w: %s", errInvalidBlobHash, shortHash(hash))
+		}
+
 		blobIDToHash[id] = hash
 		blobByHash[hash] = blob
 	}