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: it parses the age key once and reads both the database (still
streamed to a temp file) and every blob through blobgen.NewReader.

The second, blob-ID hash step is now one exported function,
blobgen.DoubleSHA256, called by the three former copies. Writer.Sum256
(the double hash) becomes Writer.ContentID so it no longer shares the
name Sum256 with Reader.Sum256 (the single plaintext hash). CompressData
and CompressStream, unused outside tests, are deleted.

Delete the unused, never-adopted secret/config newtypes in internal/types
(the redacting AgeSecretKey and AWSSecretAccessKey plus AgeRecipient,
S3Endpoint, BucketName, S3Prefix, AWSRegion, AWSAccessKeyID). Delete the
uncalled CleanupIncompleteSnapshots (and deleteSnapshot, its only caller,
now dead) and correct ARCHITECTURE.md. The only multi-recipient test
moves to blobgen; the pre-#131 encrypted-bytes hash test is removed.

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
 }