[PR] stream: add gather operator and tighten coverage [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin opened a new pull request, #2824:
URL: https://github.com/apache/pekko/pull/2824

   ## Motivation
   
   Add the `gather` operator across the Scala and Java DSLs, tighten its execution semantics, and broaden its statefulMap-equivalent regression coverage.
   
   This PR also includes the hot-path and backpressure fixes found during review, plus benchmark and documentation support for the new operator.
   
   ## Modification
   
   - add `gather` API support in the Scala and Java DSLs
   - implement the `Gather` stage and supporting `Gatherer` / `GatherCollector` APIs
   - add Scala, Java, and docs examples for the operator
   - add `FlowGatherSpec` plus Java parity coverage
   - add JMH coverage for `zipWithIndex`-style workloads
   - optimize the public single-output gather hot path
   - fix the `OneToOneGatherer` backpressure issue found during review
   - expand tests toward statefulMap-equivalent scenarios such as happy-path state retention, restart, stop, and backpressure
   
   ## Result
   
   - `gather` is documented and available end-to-end in Scala and Java DSLs
   - correctness coverage is materially stronger, especially around supervision and backpressure
   - local verification passed:
     - `sbt --no-colors 'scalafmtAll' 'stream-tests/testOnly org.apache.pekko.stream.scaladsl.FlowGatherSpec' 'stream-tests/testOnly org.apache.pekko.stream.javadsl.FlowTest' 'stream-tests/testOnly org.apache.pekko.stream.javadsl.SourceTest' 'docs/test:compile' 'bench-jmh/Jmh/compile'`
   - corrected local JMH indicates `gather`-based `zipWithIndex` is now near parity with `statefulMap`
   
   ## References
   
   - `stream/src/main/scala/org/apache/pekko/stream/impl/fusing/Ops.scala`
   - `stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowGatherSpec.scala`
   - `stream/src/main/scala/org/apache/pekko/stream/scaladsl/Gather.scala`
   - `stream/src/main/scala/org/apache/pekko/stream/javadsl/Gather.scala`
   


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Re: [PR] stream: add gather operator and tighten coverage [pekko]

["He-Pin (via GitHub)" <gi...@apache.org>]

He-Pin closed pull request #2824: stream: add gather operator and tighten coverage
URL: https://github.com/apache/pekko/pull/2824


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