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

crates/soundgen-runtime/src/bank.rs

@@ -132,6 +132,7 @@ mod tests {
                 frequency: FrequencyAutomation::fixed(440.0),
                 envelope: None,
                 filter: None,
+                vibrato: None,
                 volume: 0.5,
                 pan: 0.0,
             }],
@@ -156,6 +157,7 @@ mod tests {
                 frequency: FrequencyAutomation::fixed(440.0),
                 envelope: None,
                 filter: None,
+                vibrato: None,
                 volume: 0.5,
                 pan: 0.0,
             }],
@@ -182,6 +184,7 @@ mod tests {
                 frequency: FrequencyAutomation::fixed(440.0),
                 envelope: None,
                 filter: None,
+                vibrato: None,
                 volume: 1.0,
                 pan: 0.0,
             }],