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.

DONE.md

@@ -505,3 +505,14 @@ is in the working tree. Bodies still idle-stand at those spots.
 
 - [x] Sit pose at `steps_end`. Photo: brief raise of the influencer/tourist prop. Performer
       stays put and plays the hat/sign.
+
+## Audio
+
+Completed 2026-10-31 (working tree). Headless **134/134**; `src/test/smoke_audio.gd` checks the bed follows mood and the radio crackles, headless and windowed (real audio server).
+
+
+`AudioFeedback` is sine beeps. DONE called that "crowd ambience"; it is not.
+
+- [x] Loop a low crowd bed that rises with `CrowdSystem.mood`.
+- [x] Radio open/close crackle on the existing radio panel, still headless-safe (no-op without
+      an audio server).