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.

TODO.md

@@ -1,18 +1,10 @@
 # TODO
 
-Godot 4.3 prototype. Release **v0.2.7** is tagged. Headless suite **127/127**, plus the session,
-PLAN-verb, crowd, emergency, timeline, and windowed focus/radial smokes.
+Godot 4.3 prototype. Release **v0.2.8** is tagged. Headless suite **134/134**, plus the session,
+PLAN-verb, crowd, emergency, timeline, audio, and windowed focus/radial smokes.
 
 Host `REQUIRE_SIGNIN_VIEW` stays deferred.
 
-## Audio
-
-`AudioFeedback` is sine beeps. DONE called that "crowd ambience"; it is not.
-
-- [ ] Loop a low crowd bed that rises with `CrowdSystem.mood`.
-- [ ] Radio open/close crackle on the existing radio panel, still headless-safe (no-op without
-      an audio server).
-
 ## Accessibility
 
 - [ ] Escalation colors that still read in grayscale (shape or label, not hue alone).