Add procedural crowd ambience and radio crackle

- AudioFeedback.start_crowd_bed() builds a 4 s looping AudioStreamWAV at runtime
  (one-pole filtered noise plus a 68/103 Hz drone, seeded) and set_crowd_level()
  maps crowd mood 0..1 to -34..-12 dB, so the street gets louder as it gets
  agitated or panics. The bed starts with the shift and stops at shift end.
- play_radio_open()/play_radio_close() push short filtered-noise bursts; wired to
  opening the radio panel, submitting a report, cancelling, and closing the panel.
- Still asset-free and headless-safe: with a dummy audio server the calls no-op.
  _free_player() returns null before _ready and callers handle it.

Tests/docs
- Headless suite 127 -> 134 (test_audio_feedback.gd).
- New smoke_audio.gd: bed playing, level follows mood events, radio crackles once
  per open/close, bed stops at shift end. Verified windowed (real audio server).
- README, TODO section moved to DONE, MEMORY. Version 0.2.8.

src/game/game_manager.gd

@@ -233,6 +233,9 @@ func _reset_shift_state() -> void:
 ## Begin (or resume) the shift clock and recapture the mouse.
 func _start_shift() -> void:
 	_shift_running = true
+	if audio:
+		audio.start_crowd_bed()
+		audio.set_crowd_level(coordinator.crowd.mood)
 	coordinator.start()
 	if hud:
 		hud.set_shift_state(coordinator, selected_scenario)
@@ -322,6 +325,8 @@ func _build_ui() -> void:
 	radio_panel.submitted.connect(_on_radio_submitted)
 	radio_panel.cancelled.connect(func():
 		radio_panel.hide()
+		if audio:
+			audio.play_radio_close()
 		hud.set_action_hint("")
 		_refresh_freeze())
 	shift_review_panel = ShiftReview.new()
@@ -566,6 +571,8 @@ func _connect_crowd() -> void:
 func _on_crowd_changed(payload: Dictionary) -> void:
 	var event_type := str(payload.get("event", ""))
 	var panic := bool(payload.get("panic", false))
+	if audio:
+		audio.set_crowd_level(float(payload.get("mood", 0.0)))
 	# Panic wins: people clear away from the entrance.
 	if panic or event_type == "crowd_panic":
 		_flee_crowd()
@@ -609,6 +616,8 @@ func _responder_profile() -> NpcProfile:
 
 func _on_shift_ended(review: Dictionary) -> void:
 	_shift_running = false
+	if audio:
+		audio.stop_crowd_bed()
 	# Persist the finished shift locally (Save autoload, user:// JSON).
 	if Save:
 		Save.save_shift_result(review)
@@ -801,6 +810,8 @@ func _close_top_modal() -> bool:
 		return true
 	if radio_panel and radio_panel.visible:
 		radio_panel.hide()
+		if audio:
+			audio.play_radio_close()
 		hud.set_action_hint("")
 		_refresh_freeze()
 		return true
@@ -1126,9 +1137,13 @@ func _open_radio(emergency: bool) -> void:
 		return
 	radio_panel.open(controller, emergency, current_npc_truth)
 	hud.set_action_hint("Fill in the radio report on screen.")
+	if audio:
+		audio.play_radio_open()
 	_refresh_freeze()
 
 func _on_radio_submitted(report: Dictionary, emergency: bool) -> void:
+	if audio:
+		audio.play_radio_close()
 	hud.set_action_hint("")
 	_refresh_freeze()
 	var result: Dictionary = coordinator.radio.request_emergency(report) if emergency else coordinator.radio.request_assistance(report)