fix(audio): a software host keeps a volume set before its audio output exists (AE#660)

Sources/AetherEngine/Audio/AudioPlaybackHost.swift

@@ -145,6 +145,7 @@ final class AudioPlaybackHost {
         self.audioDecoder = aDec
         self.audioStreamIndex = resolvedAudioIdx
         self.audioOutput = AudioOutput()
+        self.audioOutput?.volume = volume
 
         if let start = startPosition, start > 0 {
             // #254: same off-main, deadline-bounded reposition the transport seek uses. Also load()'s
@@ -216,6 +217,7 @@ final class AudioPlaybackHost {
     }
 
     var clockRateForTesting: Float? { audioOutput?.rate }
+    var outputVolumeForTesting: Float? { audioOutput?.volume }
     #endif
 
     func pause() {
@@ -320,9 +322,9 @@ final class AudioPlaybackHost {
         isReady = false
     }
 
-    var volume: Float {
-        get { audioOutput?.volume ?? 1.0 }
-        set { audioOutput?.volume = newValue }
+    /// #660: held here, not only on the output, because the engine sets it before `load()` builds one.
+    var volume: Float = 1.0 {
+        didSet { audioOutput?.volume = volume }
     }
 
     // MARK: - Demux loop

Sources/AetherEngine/Native/SoftwarePlaybackHost.swift

@@ -933,6 +933,7 @@ final class SoftwarePlaybackHost {
 
         // AudioOutput owns the AVSampleBufferRenderSynchronizer (master clock). Created unconditionally: video-only previously got no clock (frozen frame, currentTime=0). Layer attached in play() after the engine hangs it in the view hierarchy (attaching free-floating fails FigVideoQueueRemote -12080 on tvOS 26+).
         self.audioOutput = AudioOutput()
+        self.audioOutput?.volume = volume
         self.audioOutput?.setPresentationOffset(seconds: audioDelaySeconds)   // AE#464
 
         // Reset the live feeder state for the new session.
@@ -1620,9 +1621,9 @@ final class SoftwarePlaybackHost {
         isVideoReadyForDisplay = false
     }
 
-    var volume: Float {
-        get { audioOutput?.volume ?? 1.0 }
-        set { audioOutput?.volume = newValue }
+    /// #660: held here, not only on the output, because the engine sets it before `load()` builds one.
+    var volume: Float = 1.0 {
+        didSet { audioOutput?.volume = volume }
     }
 
     // MARK: - Demux loop

Tests/AetherEngineTests/Issue660SoftwareVolumeTests.swift

@@ -0,0 +1,53 @@
+import Testing
+import Foundation
+@testable import AetherEngine
+
+/// #660: the engine hands its stored volume to a host right after building it, before `load()`.
+/// The FFmpeg hosts forwarded that write to an `AudioOutput` that `load()` had not built yet, so it
+/// was dropped and every software session started at full volume. The host now keeps the volume
+/// itself and hands it to each output it builds.
+@Suite("Software hosts keep a volume set before their audio output exists (#660)")
+struct Issue660SoftwareVolumeTests {
+
+    @MainActor
+    @Test("a software host reports the volume it was given before load")
+    func softwareHostKeepsVolumeBeforeLoad() {
+        let host = SoftwarePlaybackHost()
+        host.volume = 0.25
+        #expect(host.volume == 0.25)
+    }
+
+    @MainActor
+    @Test("an audio host reports the volume it was given before load")
+    func audioHostKeepsVolumeBeforeLoad() {
+        let host = AudioPlaybackHost()
+        host.volume = 0.25
+        #expect(host.volume == 0.25)
+    }
+
+    @MainActor
+    @Test("an audio host plays at the volume it was given before load")
+    func audioHostAppliesVolumeOnLoad() async throws {
+        let host = AudioPlaybackHost()
+        host.volume = 0.25
+        let demuxer = Demuxer()
+        try demuxer.open(reader: DataIOReader(data: makeWAV(seconds: 1)))
+        try await host.load(demuxer: demuxer, startPosition: nil, audioSourceStreamIndex: nil)
+        defer { host.stop() }
+        #expect(host.outputVolumeForTesting == 0.25)
+    }
+
+    private func makeWAV(seconds: Double) -> Data {
+        let sampleRate = 48_000, channels = 2
+        let pcm = Data(count: Int(Double(sampleRate) * seconds) * channels * 2)
+        var d = Data()
+        func str(_ s: String) { d.append(s.data(using: .ascii)!) }
+        func u32(_ v: UInt32) { withUnsafeBytes(of: v.littleEndian) { d.append(contentsOf: $0) } }
+        func u16(_ v: UInt16) { withUnsafeBytes(of: v.littleEndian) { d.append(contentsOf: $0) } }
+        str("RIFF"); u32(UInt32(36 + pcm.count)); str("WAVE")
+        str("fmt "); u32(16); u16(1); u16(UInt16(channels)); u32(UInt32(sampleRate))
+        u32(UInt32(sampleRate * channels * 2)); u16(UInt16(channels * 2)); u16(16)
+        str("data"); u32(UInt32(pcm.count)); d.append(pcm)
+        return d
+    }
+}