Fix oscillator/synth phase wraparound drift

Replace conditional-subtraction phase wrapping with a single stable wrap:
- OscillatorNode: `(phase + freq_mod/sr).rem_euclid(1.0)` — also wraps correctly
  when FM drives freq_mod negative (the old `if >= 1.0` wouldn't).
- SynthVoice: `(phase + frequency/sr).fract()`.
Repeated `if phase >= 1.0 { phase -= 1.0 }` accumulates f32 rounding error over
long-held notes, drifting the timbre.

daw-backend/src/audio/node_graph/nodes/oscillator.rs

@@ -164,11 +164,11 @@ impl AudioNode for OscillatorNode {
             output[frame * 2] = sample;       // Left
             output[frame * 2 + 1] = sample;   // Right
 
-            // Update phase once per frame
-            self.phase += freq_mod / sample_rate_f32;
-            if self.phase >= 1.0 {
-                self.phase -= 1.0;
-            }
+            // Update phase once per frame. `rem_euclid(1.0)` gives a numerically stable
+            // wraparound into [0, 1): repeated conditional subtraction accumulates f32
+            // rounding error over long-held notes (timbre drift), and FM can drive
+            // `freq_mod` negative, which a one-sided `if >= 1.0` wouldn't wrap at all.
+            self.phase = (self.phase + freq_mod / sample_rate_f32).rem_euclid(1.0);
         }
     }
 

daw-backend/src/effects/synth.rs

@@ -113,11 +113,10 @@ impl SynthVoice {
         // Simple sine wave
         let sample = (self.phase * 2.0 * PI).sin() * (self.velocity as f32 / 127.0) * 0.3;
 
-        // Update phase
-        self.phase += self.frequency / sample_rate;
-        if self.phase >= 1.0 {
-            self.phase -= 1.0;
-        }
+        // Update phase. Use `.fract()` for a numerically stable wraparound: repeated
+        // conditional subtraction accumulates f32 rounding error over long-held notes,
+        // which drifts the timbre.
+        self.phase = (self.phase + self.frequency / sample_rate).fract();
 
         self.age += 1;