This PR adds a new delay operation to the RxOps trait that inserts an artificial delay before emitting each element from the source stream. This is similar to the tap operator but adds a configurable delay.

Changes

• Added DelayOp[A] case class: A new operator that extends UnaryRx[A, A] for delaying stream elements
• Added delay method to RxOps trait: Allows chaining .delay(interval, unit) on any Rx stream
• Implemented DelayOp handling in RxRunner: Uses compat.scheduleOnce to asynchronously delay each element
• Added comprehensive tests: Demonstrates timing behavior and value preservation using AirSpec async testing
• Updated documentation: Clarified the difference between the existing timer-based Rx.delay and the new stream-based delay operation

Usage

import wvlet.airframe.rx.Rx
import java.util.concurrent.TimeUnit

// Delay each element by 100ms before emission
val rx = Rx.sequence(1, 2, 3)
  .delay(100, TimeUnit.MILLISECONDS)
  .tap(x => println(s"Received: $x"))

val c = rx.run()
// Elements will be emitted with 100ms delay between each

Key Features

• Preserves original values: Like tap, the delay operation passes through values unchanged
• Configurable timing: Supports any TimeUnit for flexible delay configuration
• Cross-platform support: Works on JVM, JS, and Native platforms
• Async implementation: Uses platform-specific scheduling for non-blocking delays
• Composable: Can be chained with other Rx operators

Distinction from existing Rx.delay

The existing Rx.delay(interval, unit) creates a timer stream that emits 0 once after a delay period. This new operation delays elements in an existing stream, making it more useful for throttling or artificial latency simulation.

Fixes #3871.

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