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-core/src/channel.rs

@@ -2,7 +2,7 @@
 //!
 //! Produces stereo samples (left, right) with volume and pan applied.
 
-use crate::effect::{Envelope, Filter, Sweep};
+use crate::effect::{Envelope, Filter, Sweep, Vibrato};
 use crate::generator::{Generator, Voice};
 use crate::voice::VoiceKind;
 
@@ -14,6 +14,7 @@ pub struct ChannelRenderer {
     initial_frequency: f32,
     filter: Option<Filter>,
     filter_sweep: Option<Sweep>,
+    vibrato: Option<Vibrato>,
     volume: f32,
     pan: f32,
 }
@@ -27,6 +28,7 @@ impl ChannelRenderer {
             initial_frequency: 440.0,
             filter: None,
             filter_sweep: None,
+            vibrato: None,
             volume: 1.0,
             pan: 0.0,
         }
@@ -58,6 +60,12 @@ impl ChannelRenderer {
         self
     }
 
+    /// Set vibrato (LFO frequency modulation).
+    pub fn with_vibrato(mut self, vibrato: Vibrato) -> Self {
+        self.vibrato = Some(vibrato);
+        self
+    }
+
     pub fn with_volume(mut self, volume: f32) -> Self {
         self.volume = volume.clamp(0.0, 1.0);
         self
@@ -83,6 +91,9 @@ impl ChannelRenderer {
         if let Some(s) = &mut self.filter_sweep {
             s.trigger();
         }
+        if let Some(v) = &mut self.vibrato {
+            v.trigger();
+        }
     }
 
     /// Release the envelope (note off).
@@ -101,11 +112,19 @@ impl ChannelRenderer {
     #[inline]
     pub fn tick(&mut self) -> (f32, f32) {
         // Update frequency from sweep
+        let mut current_freq = self.initial_frequency;
         if let Some(sweep) = &mut self.freq_sweep {
-            let freq = sweep.tick();
-            self.voice.set_frequency(freq);
+            current_freq = sweep.tick();
         }
 
+        // Apply vibrato (LFO frequency modulation)
+        if let Some(vib) = &mut self.vibrato {
+            let ratio = vib.tick();
+            current_freq *= ratio;
+        }
+
+        self.voice.set_frequency(current_freq);
+
         // Tick voice
         let mut sample = self.voice.tick();
 
@@ -148,6 +167,9 @@ impl ChannelRenderer {
         if let Some(f) = &mut self.filter {
             f.reset();
         }
+        if let Some(v) = &mut self.vibrato {
+            v.reset();
+        }
     }
 }