Improve draft BGM loops

docs/ROADMAP.md

@@ -139,7 +139,7 @@
 - Battlefield presentation. Maps can draw a high-resolution background under translucent terrain overlays, with terrain-specific detail patterns, connected road segments, shoreline/castle/forest/hill edge blends, and generic-enemy token marks.
 - Battle HUD presentation. Selected and hovered units now summarize battlefield role, movement type, current terrain move cost, defense/avoid bonuses, core stats, status effects, and gear so class identity reads alongside the visual tokens.
 - Battle effects. First floating combat text now covers damage, recovery, misses, support effects, poison ticks, item and gold pickups, AI target-focus changes, level-ups, and promotions. Hover target preview badges summarize attacks, tactics, and items, and the board can overlay enemy threat ranges with physical damage and hostile tactic hints.
-- Music and sound. Basic placeholder BGM and SFX now play for menus, battle flow, unit actions, tactics, items, and result states.
+- Music and sound. Longer procedural draft BGM and basic SFX now play for menus, battle flow, unit actions, tactics, items, and result states.
 - Chapter UI, objective presentation, and visual novel scenes. Pre-battle briefing now uses campaign chapter ranges for chapter-aware numbering, has a chapter progress/replay overview plus objective/progress/risk tracker, active battle HUD shows the same live objective panel, and dialogue lines can choose left or right portrait placement in an expanded portrait dialogue panel with progress and previous-line controls.
 
 ## Milestone 6: Content Expansion