decryptStream causes unbounded memory growth

### Problem:

While using `decryptStream` to stream-decrypt large S3 objects, I noticed a memory increase roughly equal to the size of the object.

I think the issue is that `_decryptStream` returns a duplexify wrapper and internally runs a pipeline:

```
const stream = new Duplexify(parseHeaderStream, decipherStream)

pipeline(
parseHeaderStream,
verifyStream,
decipherStream,
new PassThrough(),
(err: Error) => {
if (err) stream.emit('error', err)
}
)
```

The caller reads from `decipherStream` via the duplexify wrapper. The pipeline also pushes `decipherStream`'s output into the `PassThrough`. Since nothing ever reads from that `PassThrough`, its internal buffer appears to grow without bound.

### Solution:

Replacing the `PassThrough` with a no-op `Writable` that discards chunks fixes the memory growth while still absorbing the `destroy()` call:

```
const drain = new Writable({
write(_chunk, _encoding, callback) {
callback()
},
})

pipeline(
parseHeaderStream,
verifyStream,
decipherStream,
drain,
(err: Error) => {
if (err) stream.emit('error', err)
}
)
```

I tested this change locally and the memory usage stopped increasing while streaming.

`_decryptStream` のエントリポイントから始め、`parseHeaderStream`、`verifyStream`、`decipherStream` を `PassThrough` に接続しているパイプラインを調べます。説明されているとおりに未読の出力シンクを置き換え、その後、大きな S3 オブジェクトをストリーミングして、メモリ使用量がオブジェクトのサイズに応じて増加しなくなったことを確認して検証します。

索引モデルが issue の本文から書いたものです。