Fuse restInHandler into GraphStageLogic in Intersperse

[He-Pin]

Motivation

Optimize Intersperse stage by fusing the hot-path restInHandler into GraphStageLogic, reducing per-materialization object allocation.

Modification

Before:

new GraphStageLogic(shape) with OutHandler {
  val startInHandler = new InHandler { ... }  // cold path
  val restInHandler = new InHandler { ... }   // hot path - separate object
  setHandler(in, startInHandler)
  setHandler(out, this)
}

After:

new GraphStageLogic(shape) with InHandler with OutHandler { self =>
  val startInHandler = new InHandler { ... }  // cold path - still separate
  // hot path - fused into GraphStageLogic
  override def onPush(): Unit = ...
  override def onUpstreamFinish(): Unit = ...
  setHandler(in, startInHandler)
  setHandler(out, this)
}

Result

• Eliminates one InHandler allocation per materialization on the hot path
• No boolean checks added to hot path
• Cold-path startInHandler remains as separate object (only used once)

Tests

• stream-tests/testOnly FlowIntersperseSpec: 8/8 passed

[Copilot]

Pull request overview

This PR optimizes the Intersperse stream stage implementation by fusing the hot-path restInHandler into the GraphStageLogic instance, reducing per-materialization allocations while preserving the existing cold-path handler switch for the first element.

Changes:

• Refactors Intersperse#createLogic to have the GraphStageLogic directly implement InHandler for the steady-state (hot) path.
• Keeps a dedicated startInHandler for the first-element (cold) path, then switches the inlet handler to the fused logic instance.

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[pjfanning]

lgtm