Music: six more dungeon pieces and six surface pieces with flute, harp, birdsong and wind

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

docs/ROADMAP.md

@@ -54,12 +54,17 @@ in hero waves from wave 4/6, buildable with a forge). The fortresses above groun
 fortresses (fortress.rs) in a clearing. Final missions: campaign M30 and co-op M20; free play, versus matches
 and editor random maps have the option "half above ground".
 
-## 3b. Music (next milestone)
+## 3b. Music – done (v202611-00039)
 
 Six more underground tracks and a block of six surface tracks: the same leitmotif brighter and in major, synthetic
 birdsong and wind, flute leads and harp arpeggios. The music follows the camera (surface or underground) with a
 crossfade.
 
+Built in `tools/compose_music.py` (instruments flute, bird and wind; birds and wind on two extra channels only in
+surface songs, so the older pieces stay byte for byte the same): build16-19, battle9-10, surface1-4,
+surface_battle1-2; `audio::Category::Surface` / `SurfaceBattle`, chosen in `MusicDirector` when most of the
+camera's surroundings lie above ground (4 s crossfade).
+
 ## 4. Additions
 
 1. **Random events** – earthquake (tunnels cave in), lava eruption, flood, gold vein found. Rare