playback: only attenuate a gain with no peak

A peak is the only headroom check the gain code has. When the server sends a gain without one (Navidrome does this for Opus R128 gains, which carry no peak tags by design), a positive gain was applied unchecked and the 16-bit sink saturated.

Cap a peakless gain at unity instead: attenuation still applies, but a boost needs a peak to prove it fits. The cap is unchanged when a peak is present.

Update the tests that expected a peakless boost, and cover the positive preamp that can no longer lift a peakless track and the attenuation that still applies.

app/src/test/kotlin/com/subtracks/playback/ReplayGainControllerTest.kt

@@ -53,7 +53,9 @@ class ReplayGainControllerTest {
     fun theLatestItemLeavesItsGainApplied() {
         controller.setSettings(ReplayGainSettings(ReplayGainMode.Track, 0f))
 
-        controller.applyItem(item("a", trackGain = 6.0206f))
+        controller.applyItem(item("a", trackGain = 6.0206f, trackPeak = 0.5f))
+        assertEquals(2f, applied.last(), TOLERANCE)
+
         controller.applyItem(item("b", trackGain = -6.0206f))
 
         assertEquals(0.5f, applied.last(), TOLERANCE)
@@ -62,6 +64,7 @@ class ReplayGainControllerTest {
     private fun item(
         id: String,
         trackGain: Float?,
+        trackPeak: Float? = null,
     ) = queueMediaItem(
         QueueItem(
             id = id,
@@ -69,7 +72,7 @@ class ReplayGainControllerTest {
             artist = null,
             album = null,
             coverArtId = null,
-            replayGain = ReplayGain(trackGain = trackGain),
+            replayGain = ReplayGain(trackGain = trackGain, trackPeak = trackPeak),
         ),
         local = null,
         stream = "http://server/$id",

app/src/test/kotlin/com/subtracks/playback/ReplayGainProcessorTest.kt

@@ -128,7 +128,7 @@ class ReplayGainProcessorTest {
         processor.queueInput(sixteenBit(1000))
         assertEquals(listOf(500), readShorts(processor.getOutput()))
 
-        processor.flush(streamMetadata(item("b", trackGain = 6.0206f)))
+        processor.flush(streamMetadata(item("b", trackGain = 6.0206f, trackPeak = 0.5f)))
         processor.queueInput(sixteenBit(1000))
         assertEquals(listOf(2000), readShorts(processor.getOutput()))
     }
@@ -146,6 +146,7 @@ class ReplayGainProcessorTest {
     private fun item(
         id: String,
         trackGain: Float,
+        trackPeak: Float? = null,
     ): MediaItem =
         queueMediaItem(
             QueueItem(
@@ -154,7 +155,7 @@ class ReplayGainProcessorTest {
                 artist = null,
                 album = null,
                 coverArtId = null,
-                replayGain = ReplayGain(trackGain = trackGain),
+                replayGain = ReplayGain(trackGain = trackGain, trackPeak = trackPeak),
             ),
             local = null,
             stream = "http://server/$id",

app/src/test/kotlin/com/subtracks/playback/ReplayGainTest.kt

@@ -76,11 +76,35 @@ class ReplayGainTest {
 
     @Test
     fun thePreampShiftsTheAppliedGain() {
+        assertEquals(
+            0.5f,
+            replayGainLinearGain(
+                ReplayGain(trackGain = -12.0412f),
+                ReplayGainSettings(ReplayGainMode.Track, 6.0206f),
+            ),
+            TOLERANCE,
+        )
+    }
+
+    @Test
+    fun aPositivePreampCannotBoostWithoutAPeak() {
         assertEquals(
             1f,
             replayGainLinearGain(
                 ReplayGain(trackGain = -6.0206f),
-                ReplayGainSettings(ReplayGainMode.Track, 6.0206f),
+                ReplayGainSettings(ReplayGainMode.Track, 12f),
+            ),
+            TOLERANCE,
+        )
+    }
+
+    @Test
+    fun aMissingPeakStillAppliesAnAttenuation() {
+        assertEquals(
+            0.31622776f,
+            replayGainLinearGain(
+                ReplayGain(trackGain = -10f),
+                ReplayGainSettings(ReplayGainMode.Track, 0f),
             ),
             TOLERANCE,
         )
@@ -111,12 +135,11 @@ class ReplayGainTest {
     }
 
     @Test
-    fun aMissingOrZeroPeakDoesNotCap() {
+    fun aMissingOrZeroPeakAllowsNoBoost() {
         val settings = ReplayGainSettings(ReplayGainMode.Track, 0f)
-        val expected = 3.9810717f
 
-        assertEquals(expected, replayGainLinearGain(ReplayGain(trackGain = 12f), settings), TOLERANCE)
-        assertEquals(expected, replayGainLinearGain(ReplayGain(trackGain = 12f, trackPeak = 0f), settings), TOLERANCE)
+        assertEquals(1f, replayGainLinearGain(ReplayGain(trackGain = 12f), settings), TOLERANCE)
+        assertEquals(1f, replayGainLinearGain(ReplayGain(trackGain = 12f, trackPeak = 0f), settings), TOLERANCE)
     }
 
     @Test
@@ -144,9 +167,9 @@ class ReplayGainTest {
     }
 
     @Test
-    fun aNonFinitePeakDoesNotCap() {
+    fun aNonFinitePeakAllowsNoBoost() {
         assertEquals(
-            3.9810717f,
+            1f,
             replayGainLinearGain(
                 ReplayGain(trackGain = 12f, trackPeak = Float.POSITIVE_INFINITY),
                 ReplayGainSettings(ReplayGainMode.Track, 0f),