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-visualiser/src/params.rs

@@ -0,0 +1,52 @@
+//! Synthesiser parameter model — mirrors synth-core state.
+//! Serialised as JSON for postMessage() across the AudioWorklet MessagePort.
+
+use wasm_bindgen::prelude::*;
+use serde::{Serialize, Deserialize};
+
+#[wasm_bindgen]
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct SynthParams {
+    pub osc_freq:      f32, // Hz
+    pub osc_wave:      u8,  // 0=Sine 1=Saw 2=Square 3=Triangle
+    pub filter_cutoff: f32, // Hz
+    pub filter_res:    f32, // 0.0–1.0
+    pub env_attack:    f32, // seconds
+    pub env_decay:     f32,
+    pub env_sustain:   f32, // 0.0–1.0
+    pub env_release:   f32,
+    pub lfo_rate:      f32, // Hz
+    pub lfo_depth:     f32, // 0.0–1.0
+    pub master_gain:   f32, // 0.0–1.0
+}
+
+#[wasm_bindgen]
+impl SynthParams {
+    #[wasm_bindgen(constructor)]
+    pub fn new() -> Self {
+        Self {
+            osc_freq:      440.0,
+            osc_wave:      1,
+            filter_cutoff: 2000.0,
+            filter_res:    0.3,
+            env_attack:    0.01,
+            env_decay:     0.1,
+            env_sustain:   0.7,
+            env_release:   0.3,
+            lfo_rate:      2.0,
+            lfo_depth:     0.0,
+            master_gain:   0.8,
+        }
+    }
+
+    pub fn to_json(&self) -> String {
+        // serde_json would pull in std; for WASM we use the js glue instead.
+        // This is a simple manual serialisation for the scaffold.
+        format!(
+            r#"{{"osc_freq":{:.2},"osc_wave":{},"filter_cutoff":{:.2},"filter_res":{:.3},"env_attack":{:.4},"env_decay":{:.4},"env_sustain":{:.3},"env_release":{:.4},"lfo_rate":{:.3},"lfo_depth":{:.3},"master_gain":{:.3}}}"#,
+            self.osc_freq, self.osc_wave, self.filter_cutoff, self.filter_res,
+            self.env_attack, self.env_decay, self.env_sustain, self.env_release,
+            self.lfo_rate, self.lfo_depth, self.master_gain
+        )
+    }
+}