`HashingOutputStream` should extend `FilterOutputStream`

### Problem

The `HashingOutputStream` does not properly handle `close` or `flush`. The JDK's `FilterOutputStream` is designed for subclasses that wrap another stream, and ensures that `close` and `flush` both propagate to the wrapped stream. Since the `HashingOutputStream` doesn't do this, clients that use it must explicitly close both the `HashingOutputStream` and the stream it wraps, which can be error prone and lead to more code.

### Solution

Let `HashingOutputStream` wrap `FilterOutputStream`.

### Alternatives

_No response_

Locate the HashingOutputStream implementation and inspect how it wraps its underlying stream. Compare its close and flush behavior with Java's FilterOutputStream, then verify that both operations propagate to the wrapped stream after the change.

Written by the indexing model from the issue text.