Replaces placeholder AudioManager with full implementation: floor-based
music routing via _derive_floor_from_room, cross-fade tween between
AudioStreamPlayers, SFX event-key dispatch, and room-change guard to
prevent redundant load attempts. 11 new tests (207 total, 206 passing).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Steven Wroblewski