Initial commit of a toy analogue audio generator

Key components:
- no_std core so it can be used bare metal on embedded systems
- default implementation for RPi 2350, with midi input and I2s output using PIO
- Visualiser for the web to play with on a dev system

crates/synth-core/src/oscillator.rs

@@ -0,0 +1,62 @@
+//! Voltage-controlled oscillator (VCO).
+//!
+//! Waveforms: Sine, Saw, Square, Triangle, Pulse (variable width).
+//! Uses phase accumulation; bandlimited variants (BLEP/BLAMP) to follow.
+
+use crate::{AudioProcessor, CVProcessor, config::SampleRate};
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Waveform {
+    Sine,
+    Saw,
+    Square,
+    Triangle,
+    Pulse(f32), // pulse width 0.0–1.0
+}
+
+pub struct Vco {
+    pub waveform: Waveform,
+    pub freq_hz: f32,
+    phase: f32,
+    sample_rate: SampleRate,
+}
+
+impl Vco {
+    pub fn new(sample_rate: SampleRate, freq_hz: f32, waveform: Waveform) -> Self {
+        Self { waveform, freq_hz, phase: 0.0, sample_rate }
+    }
+
+    #[inline]
+    fn next_sample(&mut self) -> f32 {
+        let p = self.phase;
+        let sample = match self.waveform {
+            Waveform::Sine     => libm::sinf(p * core::f32::consts::TAU),
+            Waveform::Saw      => 2.0 * p - 1.0,
+            Waveform::Square   => if p < 0.5 { 1.0 } else { -1.0 },
+            Waveform::Triangle => 4.0 * (p - libm::floorf(p + 0.5)).abs() - 1.0,
+            Waveform::Pulse(w) => if p < w { 1.0 } else { -1.0 },
+        };
+        let next = p + self.freq_hz * self.sample_rate.period();
+        self.phase = next - libm::floorf(next);
+        sample
+    }
+}
+
+impl<const B: usize> AudioProcessor<B> for Vco {
+    fn process(&mut self, out: &mut [f32; B]) {
+        for s in out.iter_mut() {
+            *s = self.next_sample();
+        }
+    }
+
+    fn reset(&mut self) {
+        self.phase = 0.0;
+    }
+}
+
+impl<const B: usize> CVProcessor<B> for Vco {
+    /// CV is 1 V/oct: 0 V = 440 Hz, +1 V = 880 Hz, −1 V = 220 Hz.
+    fn set_cv(&mut self, cv: f32) {
+        self.freq_hz = 440.0 * libm::powf(2.0, cv);
+    }
+}