/claim #9810

Summary

Fix ZStream.buffer(n) prefetch semantics so it doesn’t run more than n elements ahead.

• For n > 1, internal queue capacity is adjusted to n - 1 to account for the single in-flight element downstream.
• For n == 1, use a synchronous handoff (ack per element) so the upstream cannot advance beyond one element.

Proof

Tests

sbt "streamsTestsJVM/testOnly zio.stream.ZStreamSpec -- -t buffer"

Includes new regression tests:

• buffer(1) does not run more than one element ahead
• buffer(2) does not run more than two elements ahead