Soundtrack expanded to 35 pieces with a leitmotif variant generator and 8 new instruments

- generator: leitmotif as scale degrees, keys/modes, augmentation, diminution, inversion,
  fragment sequences, accompaniment and drum styles; hand-written title 2, boss themes, victory 2
- organ, bell, choir, sub bass, tom, crash, harp, timpani; 8-bit samples, unused instruments stripped
- src/tracks.rs generated with categories; music director with boss themes and room moods
- all pieces validated with libopenmpt; notes checked to stay in key

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

DESIGN.md

@@ -41,8 +41,10 @@ An original dungeon management game (genre: dungeon building & defence) that run
 ## Music & sound
 - Own **XM/MOD replayer in Rust** (`src/xm.rs`): instruments, envelopes, fadeout, auto-vibrato, linear/Amiga frequencies, common FT2 effects; verified against libopenmpt
 - **Shared mixer** for music + sound effects (`src/audio.rs`), streamed to the browser by the JS plugin (replaces macroquad audio)
-- Soundtrack in classic **Amiga chiptune** style, composed as `.xm` by `tools/compose_music.py` (editable in MilkyTracker): title/menu, **3 build tracks** (rotating), **2 battle tracks** (wave defence, gate assault), victory/defeat jingles; **the title melody is a recognisable leitmotif in every piece**
-- Music changes with **soft crossfades** (2.5 s): title → build → battle when heroes invade → storm music while attacking the gate → jingles
+- Soundtrack in classic **Amiga chiptune** style, **35 pieces**, composed as `.xm` by `tools/compose_music.py` (editable in MilkyTracker): 2 title, 15 build, 7 wave battle, 4 gate assault, 2 boss (Paladin of Light, Gate Warden), 1 torture chamber mood, 1 temple mood, 2 victory, 1 defeat; **the title melody is a recognisable leitmotif in every piece**
+- Most pieces come from a **variant generator**: the leitmotif is stored as scale degrees and realised in different keys and modes (minor, harmonic minor, dorian, phrygian, lydian), augmented, diminished, inverted or as fragment sequences, with varying accompaniment, drum styles, instruments and forms; title 2, both boss themes and the second victory jingle are hand-written
+- 16 chip instruments (pulse, square, saw pad, triangle bass, pluck, kick, snare, hihat, organ, bell, choir, sub bass, tom, crash, harp, timpani); 8-bit samples, each file only contains the instruments it uses
+- Music changes with **soft crossfades** (2.5 s): title → build (rotating through all build pieces) → battle when heroes invade → assault while attacking the gate → boss themes while a Paladin of Light or the Gate Warden is alive → jingles; near a torture chamber or temple the matching **room mood** plays
 - Sound effects are **synthesised chip sounds** (`src/sfx.rs`), positioned by distance and stereo panning
 
 ## Map size