feat(android): Sonos/UPnP queue plays leveled URLs, rendered a track ahead (M464 #5002)

Every URL the Sonos queue loader sends is minted with level=true and
the track's album-play verdict from its neighbours in the queue; the
server returns the plain stream when leveling is off or changes
nothing. The playing track and the one after it are rendered ahead,
and each time the renderer moves on, the next is.

Server: a mint no longer prerenders on its own. A queue load mints
every track, which would have started an ffmpeg render per track at
once. The request now carries prerender, and at most two prerenders
run at a time; past that they are dropped, since a fetch renders on
demand anyway.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/library/leveled_test.go

@@ -195,6 +195,41 @@ func TestLeveledRenderer_CoalescesAndCaches(t *testing.T) {
 	}
 }
 
+// Prerenders past the slot limit are dropped rather than piling up ffmpeg
+// processes; a slot frees when its render ends.
+func TestLeveledRenderer_PrerenderSlots(t *testing.T) {
+	r, renders, _ := testRenderer(t, 2048, 10)
+	gate := make(chan struct{})
+	r.render = func(_ context.Context, _, dst string, _ LeveledGain, _ leveledSource) error {
+		<-gate
+		renders.Add(1)
+		return os.WriteFile(dst, []byte("flac"), 0o600)
+	}
+	g := LeveledGain{CentiDB: -300}
+	for i := range maxLeveledPrerenders {
+		if !r.Prerender(writeSource(t, string(rune('a'+i))), g) {
+			t.Fatalf("prerender %d refused with slots free", i)
+		}
+	}
+	if r.Prerender(writeSource(t, "z"), g) {
+		t.Fatal("a prerender past the limit was accepted")
+	}
+	close(gate)
+	deadline := time.Now().Add(5 * time.Second)
+	for !r.Prerender(writeSource(t, "y"), g) {
+		if time.Now().After(deadline) {
+			t.Fatal("no slot freed after the renders finished")
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	for renders.Load() != maxLeveledPrerenders+1 {
+		if time.Now().After(deadline) {
+			t.Fatalf("renders = %d, want %d", renders.Load(), maxLeveledPrerenders+1)
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+}
+
 func TestLeveledRenderer_MissingSource(t *testing.T) {
 	r, _, _ := testRenderer(t, 2048, 10)
 	_, err := r.Path(context.Background(), LeveledSource{TrackID: "x", Path: "/nonexistent/x.flac"}, LeveledGain{CentiDB: 100})