Add vibrato support + fix feedback loop + GUI improvements

Vibrato (LFO frequency modulation):
- New VibratoSpec in SoundSpec: rate (Hz), depth (cents), delay (s)
- ChannelRenderer applies vibrato to frequency each tick
- Supported on Pulse and Triangle channels
- GUI: vibrato controls (enable, rate, depth) in channel panel
- Enables bird chirps, sirens, wobbles

Feedback loop fixes:
- generate_batch now accepts full SoundSpec array (not just names)
  so each sound is unique
- render_sound returns reference examples from feedback DB
- Fixed GUI bug: clicking stars no longer erases feedback text
- Feedback text auto-saves on Enter / focus loss
- Replaced emoji buttons (✓/🗑) with text (Save/Del)
- Green saved indicator when feedback exists in DB

99 tests passing, 0 warnings

.gitignore

@@ -6,10 +6,15 @@ release/
 # WAV output (generated, not source)
 *.wav
 /sfx_output/
+/training_sounds/
 
 # User-generated sound specs
 /sound.json
 
+# Feedback database (contains user ratings, keep local)
+/feedback.db
+/feedback_dataset.jsonl
+
 # IDE / editor
 .idea/
 .vscode/