Fix transient crashes during audio-engine recovery

Packages/ContinuityKit/Package.swift

@@ -55,5 +55,10 @@ let package = Package(
             dependencies: ["Domain", .product(name: "ContinuityCore", package: "ContinuityCore")],
             swiftSettings: [.swiftLanguageMode(.v5)]
         ),
+        .testTarget(
+            name: "PlaybackTests",
+            dependencies: ["Playback"],
+            swiftSettings: [.swiftLanguageMode(.v5)]
+        ),
     ]
 )

Packages/ContinuityKit/Sources/Playback/Player+AudioSession.swift

@@ -97,8 +97,7 @@ extension Player {
         let wasPlaying = isPlaying
         let resumePosition = position
         // Don't touch the dead engine/decks — even stop() can throw on orphaned nodes.
-        isTransitioning = false
-        transitionProgress = 0
+        clearTransitionState()
         isPlaying = false
         stopTimer()
         audio = nil
@@ -137,12 +136,25 @@ extension Player {
         resumeAfterInterruption = false
         // Observers only exist once the stack does, so `audio` is always live here.
         guard currentTrack != nil, let audio else { return }
+        let transitionWasInterrupted = isTransitioning
         guard ensureRunning() else {
+            if transitionWasInterrupted {
+                // The idle deck's schedule is no longer trustworthy. Clear the blend flags even
+                // when CoreAudio cannot restart so a later play cannot resume that stale deck.
+                clearTransitionState()
+            }
             Logger.audio.error("engine restart failed during recovery")
             isPlaying = false
             stopTimer()
             return
         }
+        if transitionWasInterrupted {
+            // Configuration-change recovery bypasses pauseForEnvironment(), but an engine stop
+            // invalidates both decks just the same. Drop the incoming deck before rescheduling
+            // the outgoing one; otherwise pause/resume can call play() on its stale schedule.
+            Logger.audio.info("engine stop interrupted transition — discarding incoming deck")
+            cancelTransition()
+        }
         // Engine stops invalidate node schedules — reschedule from where the clock stood.
         if audio.current.seekRealFile(to: position) {
             baselineSeconds = position

Packages/ContinuityKit/Sources/Playback/Player+Transitions.swift

@@ -174,9 +174,19 @@ extension Player {
             audio.current.vocalsGain = 1
             audio.current.bassGainDB = 0
         }
+        clearTransitionState()
+    }
+
+    /// Clears blend bookkeeping without touching AVAudio objects. Media-services reset leaves
+    /// every node orphaned, so that path must abandon state without calling even `stop()`.
+    func clearTransitionState() {
         isTransitioning = false
+        transitionTargetIndex = currentIndex
         activeTransitionDurationSeconds = 0
         isUserInitiatedSkipTransition = false
+        incomingStartOffset = 0
+        incomingPitchShiftSemitones = 0
+        incomingRate = 1
         transitionProgress = 0
     }
 }

Packages/ContinuityKit/Tests/PlaybackTests/PlayerTransitionRecoveryTests.swift

@@ -0,0 +1,30 @@
+import XCTest
+@testable import Playback
+
+@MainActor
+final class PlayerTransitionRecoveryTests: XCTestCase {
+    func testClearingTransitionStateDoesNotMaterializeAudioStack() {
+        let player = Player()
+        player.currentIndex = 2
+        player.transitionTargetIndex = 3
+        player.activeTransitionDurationSeconds = 5
+        player.isUserInitiatedSkipTransition = true
+        player.incomingStartOffset = 1.5
+        player.incomingPitchShiftSemitones = 1
+        player.incomingRate = 1.05
+        player.transitionProgress = 0.4
+        player.isTransitioning = true
+
+        player.clearTransitionState()
+
+        XCTAssertFalse(player.isTransitioning)
+        XCTAssertEqual(player.transitionTargetIndex, player.currentIndex)
+        XCTAssertEqual(player.activeTransitionDurationSeconds, 0)
+        XCTAssertFalse(player.isUserInitiatedSkipTransition)
+        XCTAssertEqual(player.incomingStartOffset, 0)
+        XCTAssertEqual(player.incomingPitchShiftSemitones, 0)
+        XCTAssertEqual(player.incomingRate, 1)
+        XCTAssertEqual(player.transitionProgress, 0)
+        XCTAssertNil(player.audio)
+    }
+}