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/blob/packer.go

@@ -487,7 +487,7 @@ func (p *Packer) closeBlobWriter() (string, int64, error) {
 		return "", 0, fmt.Errorf("seeking to start: %w", err)
 	}
 
-	finalHash := p.currentBlob.writer.Sum256()
+	finalHash := p.currentBlob.writer.ContentID()
 
 	return hex.EncodeToString(finalHash), finalSize, nil
 }