add SkipMode enum (UseReverbTail / CrossFade / Jump)

advance_to_next_song / advance_to_track now branch on the new SkipMode export:

- UseReverbTail (default): existing behavior. Accelerate the outgoing song so
  remaining time hits ReverbTail, both songs play during the overlap, outgoing
  stops naturally.
- CrossFade: programmatic volume crossfade. Outgoing fades from current volume
  to silent over CrossFadeDuration seconds (then stops), incoming starts silent
  and fades up to full over the same duration. Independent of any global fade.
- Jump: hard cut. Outgoing song's streams are stopped immediately and removed
  from the active list before the incoming starts.

For CrossFade we needed per-song fade state (the global _fade_db can't drive
two songs in opposite directions at once), so _ActiveSong gained fade_db /
fade_from_db / fade_to_db / fade_duration / fade_elapsed / fading /
stop_after_fade fields. _apply_volumes_to_song now adds both the global and
per-song fade_db to volume_db; process() advances each song's fade per-frame
and stops songs whose fades reach the silent target with stop_after_fade set.

CrossFadeDuration is hidden in the inspector via _validate_property unless
SkipMode == CrossFade. The natural end-of-song advance is unchanged — it still
uses the song's reverb tail. SkipMode only affects programmatic skips.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -65,6 +65,8 @@ Plays through a playlist one song at a time, blending three intensity layers per
 | `Playlist` | `DynamicSoundPlaylist` | `null` | Songs to play in order. Read-only as songs play — the player tracks an internal index. |
 | `Intensity` | `float` (0–1) | `0` | Runtime mix between the three layers. `0` = layer 1 dominates, `1` = layer 3 dominates. |
 | `LoopMode` | enum | `All` | `Single` keeps repeating the current song. `All` advances through the playlist and wraps back to index 0 after the last song. Both modes loop forever; there's no "play once and stop" mode. |
+| `SkipMode` | enum | `UseReverbTail` | How `advance_to_next_song()` / `advance_to_track()` transition. `UseReverbTail` accelerates the outgoing song so its remaining time hits `ReverbTail` and the new song plays on top — relies on the song's audio having a natural tail. `CrossFade` programmatically fades the outgoing song down and the incoming song up over `CrossFadeDuration` seconds. `Jump` is a hard cut. Does **not** affect natural end-of-song advancement (that always uses reverb tail). |
+| `CrossFadeDuration` | `float` | `1.0` | Duration of the volume crossfade in seconds when `SkipMode = CrossFade`. Hidden in the inspector for the other modes. |
 
 When the node enters the tree in the editor, `stream` is auto-assigned an `AudioStreamPolyphonic` and `bus` is set to `"Music"` if a bus by that name exists in the project (otherwise it's left at `"Master"`). User-customised buses are not overridden.
 
@@ -79,7 +81,7 @@ When the node enters the tree in the editor, `stream` is auto-assigned an `Audio
 | `fade_in(duration: float)` | Starts playback (if needed), unpauses, and ramps volume from silent to `volume_db` over `duration` seconds. Works regardless of `autoplay`. |
 | `fade_out(duration: float)` | Ramps volume from current level to silent over `duration` seconds, then stops the polyphonic session. A subsequent `fade_in` reinitialises. |
 | `get_current_song_index() -> int` | Index of the song most recently kicked off in `Playlist.QueuedSongs`. During a reverb-tail crossover this becomes the incoming song the moment it starts, even though the outgoing song is still audible. |
-| `advance_to_next_song()` | Skip to the next song in the playlist (wraps from the end back to index 0). Triggers a smooth transition through the current song's reverb tail — accelerates the outgoing song so its remaining time hits `ReverbTail` immediately, then starts the new song on top. With `ReverbTail = 0` this is effectively a hard cut. |
+| `advance_to_next_song()` | Skip to the next song in the playlist (wraps from the end back to index 0). Transition style is controlled by `SkipMode`. |
 | `advance_to_track(index: int)` | Same as `advance_to_next_song`, but jumps to a specific index. Out-of-range indices are ignored. |
 
 #### Behaviour