Add Transitioning check to DelayAltMusic in Level.LoadLevel

[SilverDorian46]

to preface, rooms have an additional attribute called delayAltMusicFade (in Lönn, it's aliased to delayAlternativeMusicFade), which works with the room's alt music (set with the alt_music attribute, which in Lönn is aliased to musicAlternative).
when alt music is played, the main music is muted. so in order to be heard, alt music has to be routed differently to main music. the base game has them routed to music/tunes/side_rooms.

the value of delayAltMusicFade is assigned to the LevelData's DelayAltMusic field, which is a boolean that determines at which moment to play or stop the alt music.
if it's false, as it is by default, the alt music is played or stopped as soon as the room is loaded.
if it's true, then that is delayed for when the transition into the next room is complete.

currently, due to an oversight, the true setting makes it only work during room transitions.
so what happens when you "Save and Quit" from that room, and then load the save again and return to it?
the main music will play, but the alt music won't, until you enter the room again via transition.

this PR attempts to fix this by adding a check for whether the level's transitioning routine is being played.

a base game example of an alt music room with the delay enabled is the secret room in Chapter 6.
for comparison, an example of an alt music room without the delay is Theo's camp in Chapter 1.

[microlith57]

i don't really understand the impact of the bug here. can you provide examples of reproduction steps with expected vs actual behaviour?

[SilverDorian46]

i don't really understand the impact of the bug here. can you provide examples of reproduction steps with expected vs actual behaviour?

Reproduction steps:

1. enter vanilla Chapter 6 (Reflection)
2. go to the secret room (that's on the top left side) - that room has alt music and delayAltMusicFade enabled
3. Save and Quit
4. reload save - that brings you back to the room

Expected behaviour:
alt music plays, and main music is muted

Actual behaviour:
alt music doesn't play, and main music is heard. however, reentering the room via transition makes alt music play as expected

[microlith57]

LGTM

[SilverDorian46]

oh yeah, should we tell the Celeste developers about this? it is a bug in the base game after all

[maddie480-bot]

The pull request was approved and entered the 3-day last-call window. Since no PR should be merged within 3 days of the next rolling release, the last-call window is extended further.
If no further reviews happen, it will end on Mar 8, 2026, 12:00 AM UTC, after which the pull request will be able to be merged.

[maddie480-bot]

The last-call window for this pull request ended. It can now be merged if no blockers were brought up.