/claim #9810

Proposed changes

Fixes #9810.

This PR fixes an off by one behavior in ZStream.buffer(capacity) where buffer(n) allowed n + 1 elements in flight.

1. capacity <= 0
   No-op. Returns the original stream unchanged.

2. capacity == 1
   Uses a synchronous ZStream.Handoff path so exactly one element is buffered ahead.

3. capacity >= 2
   Uses internal queue size capacity - 1 so queue capacity plus one in-flight element equals exactly capacity.

Tests added

1. buffer(1) buffers exactly 1 element
2. buffer(2) buffers exactly 2 elements
3. buffer(0) is a no-op

These tests use Promise coordination to make assertions deterministic.

Validation

• sbt "-Dsbt.supershell=false" "streamsTestsJVM/testOnly zio.stream.ZStreamSpec -- -t buffer"
• sbt "-Dsbt.supershell=false" "streamsTestsJS/testOnly zio.stream.ZStreamSpec -- -t buffer"
• Native run with Windows toolchain env setup, then:
  • sbt "-Dsbt.supershell=false" "streamsTestsNative/testOnly zio.stream.ZStreamSpec -- -t buffer"

Result:
23 tests passed, 0 failed, 0 ignored on each platform run.

Proof

https://github.com/user-attachments/assets/d789aecb-7644-4812-851d-1b1975bc0125

Demo video attached showing:

1. git diff for the two changed files
2. realtime JVM, JS, and Native buffer test runs
3. passing test summaries