Replace Calculator placeholder with tests against real radioplayer types

[Copilot]

The :tests subproject existed purely as a structural demo with an unrelated Calculator class. It now imports :radioplayer and tests the actual public API.

Constraint

PlatformMediaPlayer is an expect class with actual implementations only for Android and iOS. Adding a JVM target to :tests would fail to compile without a JVM actual.

Changes

• radioplayer — adds jvm() target + a JVM stub actual class PlatformMediaPlayer that fires subscribeState/subscribeProgress callbacks synchronously and exposes currentState and emitProgress() for test assertions
• tests/build.gradle.kts — adds projects.radioplayer as a commonTest dependency
• tests/commonTest — replaces CalculatorTest with four focused test classes:
  • ProgressTest — data class equality, copy, elapsed/duration ratio
  • PlaybackStateTest — all 5 enum values, names, count
  • MediaPlayerItemTest — live station and on-demand track implementations
  • PlatformMediaPlayerTest — state transitions via subscribeState (common API only)
• tests/jvmTest — PlatformMediaPlayerJvmTest exercises JVM-stub extras (currentState, emitProgress)

// commonTest — uses only the expect class surface
@Test
fun testStopTransitionsToStoppedState() {
    val player = PlatformMediaPlayer()
    val states = mutableListOf<PlaybackState>()
    player.subscribeState { states += it }
    player.play()
    player.stop()
    assertEquals(listOf(PlaybackState.PLAYING, PlaybackState.STOPPED), states)
}

// jvmTest — uses the stub's extra helpers
@Test
fun testSubscribeProgressReceivesEmittedUpdates() {
    val player = PlatformMediaPlayer()
    val received = mutableListOf<Progress>()
    player.subscribeProgress { received += it }
    player.emitProgress(Progress(elapsed = 10.0, duration = 180.0))
    assertEquals(10.0, received[0].elapsed)
}

Run with ./gradlew :tests:jvmTest.

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