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/audio_feedback.gd

@@ -8,6 +8,11 @@ extends Node
 const SAMPLE_RATE := 22050.0
 
 var _players: Array[AudioStreamPlayer] = []
+var _bed_player: AudioStreamPlayer = null
+## Observable state so headless tests and the HUD can see what was requested.
+var crowd_level: float = 0.0
+var bed_playing := false
+var radio_crackles := 0
 
 func _ready() -> void:
 	# Pre-create a few players so overlapping cues don't cut each other off.
@@ -30,6 +35,43 @@ func play_escalation(level: int) -> void:
 	var freq := 440.0 - float(level) * 40.0
 	_play_tone(maxf(freq, 180.0), 0.18, 0.6)
 
+## Start the looping crowd bed (low murmur). Called once per shift; safe to call
+## again. Volume then follows `set_crowd_level`.
+func start_crowd_bed() -> void:
+	if _bed_player == null:
+		_bed_player = AudioStreamPlayer.new()
+		_bed_player.name = "CrowdBed"
+		add_child(_bed_player)
+		_bed_player.stream = _build_crowd_stream()
+		_bed_player.volume_db = _level_to_db(crowd_level)
+	_bed_player.play()
+	bed_playing = true
+
+func stop_crowd_bed() -> void:
+	if _bed_player:
+		_bed_player.stop()
+	bed_playing = false
+
+## Crowd mood (0..1) drives the bed volume: quiet when calm, louder when the
+## street is agitated or panicking.
+func set_crowd_level(mood: float) -> void:
+	crowd_level = clampf(mood, 0.0, 1.0)
+	if _bed_player:
+		_bed_player.volume_db = _level_to_db(crowd_level)
+
+static func _level_to_db(mood: float) -> float:
+	return lerpf(-34.0, -12.0, clampf(mood, 0.0, 1.0))
+
+## Radio open cue: a short burst of filtered crackle.
+func play_radio_open() -> void:
+	radio_crackles += 1
+	_play_noise(0.09, 0.35)
+
+## Radio close cue: a lower, shorter click.
+func play_radio_close() -> void:
+	radio_crackles += 1
+	_play_noise(0.06, 0.25, 1400.0)
+
 ## Play a denial/error cue (low buzz).
 func play_deny() -> void:
 	_play_tone(220.0, 0.20, 0.5)
@@ -53,7 +95,64 @@ func _play_tone(freq: float, duration: float, volume: float, delay: float = 0.0)
 		var sample := sin(TAU * freq * t) * 0.6
 		playback.push_frame(Vector2(sample, sample))
 
+## Deterministic low murmur for the crowd bed: filtered noise plus a low drone.
+func _build_crowd_stream() -> AudioStreamWAV:
+	const SECONDS := 4.0
+	var frames := int(SAMPLE_RATE * SECONDS)
+	var data := PackedByteArray()
+	data.resize(frames * 2)
+	var rng := RandomNumberGenerator.new()
+	rng.seed = 20261031
+	var smooth := 0.0
+	for i in frames:
+		var t := float(i) / SAMPLE_RATE
+		# Low-passed noise (a simple one-pole filter) sounds like distant talking.
+		smooth = lerpf(smooth, rng.randf_range(-1.0, 1.0), 0.02)
+		var drone := sin(TAU * 68.0 * t) * 0.25 + sin(TAU * 103.0 * t) * 0.12
+		var sample := (smooth * 2.2 + drone) * 0.35
+		var v := int(clampf(sample, -1.0, 1.0) * 32767.0)
+		data[i * 2] = v & 0xFF
+		data[i * 2 + 1] = (v >> 8) & 0xFF
+	var stream := AudioStreamWAV.new()
+	stream.format = AudioStreamWAV.FORMAT_16_BITS
+	stream.mix_rate = int(SAMPLE_RATE)
+	stream.stereo = false
+	stream.data = data
+	stream.loop_mode = AudioStreamWAV.LOOP_FORWARD
+	stream.loop_begin = 0
+	stream.loop_end = frames
+	return stream
+
+## Short filtered noise burst used for the radio crackle.
+func _play_noise(duration: float, volume: float, cutoff_hz: float = 3200.0) -> void:
+	var player := _free_player()
+	if player == null:
+		return
+	var frames := int(SAMPLE_RATE * duration)
+	var data := PackedByteArray()
+	data.resize(frames * 2)
+	var rng := RandomNumberGenerator.new()
+	rng.seed = 7
+	var smooth := 0.0
+	var alpha := clampf(cutoff_hz / SAMPLE_RATE, 0.01, 0.9)
+	for i in frames:
+		smooth = lerpf(smooth, rng.randf_range(-1.0, 1.0), alpha)
+		var env := 1.0 - float(i) / float(maxi(frames, 1))
+		var v := int(clampf(smooth * env, -1.0, 1.0) * 32767.0)
+		data[i * 2] = v & 0xFF
+		data[i * 2 + 1] = (v >> 8) & 0xFF
+	var stream := AudioStreamWAV.new()
+	stream.format = AudioStreamWAV.FORMAT_16_BITS
+	stream.mix_rate = int(SAMPLE_RATE)
+	stream.stereo = false
+	stream.data = data
+	player.stream = stream
+	player.volume_db = linear_to_db(volume)
+	player.play()
+
 func _free_player() -> AudioStreamPlayer:
+	if _players.is_empty():
+		return null
 	for player in _players:
 		if not player.playing:
 			return player