Working with long-lived streams

[sumimakito]

When processing grapheme clusters from a long-lived stream, the final cluster in a sequence of writes can remain buffered indefinitely, as the stream never closes to trigger a flush. This has caused issues in downstream use cases.

Therefore, for long-lived streams, we must implement a mechanism to flush the buffer. Two potential solutions are:

1. A signal mechanism to explicitly trigger a flush
2. A timeout mechanism to flush the buffer after a period of inactivity