Replace 3 direct GameState.dialogue_active mutations and all
AudioManager.apply_dip/clear_dip calls with signals:
dialogue_state_changed, audio_dip_requested, audio_dip_cleared.
dialogue_box.gd now has zero references to GameState or AudioManager.
main.gd wires coordinator handlers in _ready() (D-020).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>