Remove the unused crypto path and write the blob-ID hash step once (closes #151)

Production encryption and decryption already run through blobgen; the crypto package (Encryptor, Decryptor, UpdateRecipients, the fx Module) and Vaultik.GetEncryptor/GetDecryptor had no production caller. Delete crypto and route verify --deep through the same blobgen reader restore uses, parsing the age key once.

The second blob-ID hash step is now one exported blobgen.DoubleSHA256; Writer.Sum256 (the double hash) becomes Writer.ContentID so it no longer collides with Reader.Sum256 (the single plaintext hash). Also delete the never-adopted internal/types newtypes and the uncalled CleanupIncompleteSnapshots and its now-dead deleteSnapshot caller, and correct ARCHITECTURE.md. No production behavior changes.

Model: opus-4-8

internal/vaultik/snapshot.go

@@ -56,8 +56,8 @@ func (v *Vaultik) CreateSnapshot(opts *SnapshotCreateOptions) error {
 		return err
 	}
 
-	// Clean up incomplete snapshots FIRST, before any scanning
-	// This is critical for data safety - see CleanupIncompleteSnapshots for details
+	// Clean up incomplete snapshots FIRST, before any scanning.
+	// This is critical for data safety; PruneDatabase below does it.
 	hostname := v.Config.Hostname
 	if hostname == "" {
 		hostname, _ = os.Hostname()