Holiday music: a dark carol for Christmas, a spring tune for Easter, a harpsichord dance for Halloween

During a holiday the title music plays its piece, the build music every fourth time.

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

src/main.rs

@@ -492,11 +492,12 @@ fn apply_settings(s: &Settings, r3d: &mut R3d) {
 struct MusicDirector {
     next: std::collections::HashMap<u8, usize>, // category -> next index within the category
     current: Option<audio::Category>,
+    build_picks: usize, // build tracks picked so far (during a holiday every fourth is its piece)
 }
 
 impl MusicDirector {
     fn new() -> MusicDirector {
-        MusicDirector { next: std::collections::HashMap::new(), current: None }
+        MusicDirector { next: std::collections::HashMap::new(), current: None, build_picks: 0 }
     }
 
     fn pick(&mut self, c: audio::Category) -> usize {
@@ -573,11 +574,17 @@ impl MusicDirector {
         }
         let rotate = matches!(want, C::Build | C::Title | C::Surface) && self.current == Some(want) && audio::music_loops() >= 2;
         // after the chosen track was let go, the director picks again
-        let foreign = chosen.is_none() && audio::current_track().map_or(false, |t| crate::tracks::TRACKS.get(t).map_or(false, |x| x.1 != want)) && calm && self.current == Some(want);
+        // (a holiday's piece belongs to the title and build music)
+        let holiday = holiday::current();
+        let fits = |c: C| c == want || (matches!(want, C::Title | C::Build) && audio::holiday_music(holiday) == Some(c));
+        let foreign = chosen.is_none() && audio::current_track().map_or(false, |t| crate::tracks::TRACKS.get(t).map_or(false, |x| !fits(x.1))) && calm && self.current == Some(want);
         if self.current != Some(want) || rotate || foreign || audio::current_track().is_none() {
             let before = self.current;
             self.current = Some(want);
-            let t = self.pick(want);
+            let t = self.pick(audio::holiday_category(want, holiday, self.build_picks));
+            if want == C::Build {
+                self.build_picks += 1;
+            }
             // between the dungeon and the surface the music changes slowly
             let fade = if matches!(want, C::Victory | C::Defeat) {
                 1.0