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/patch.rs

@@ -0,0 +1,40 @@
+//! Signal routing — connects module outputs to inputs.
+//!
+//! A `Patch` is a fixed-size connection graph. Each cable routes
+//! the output buffer of one slot into the CV or audio input of another.
+//! Const generics keep everything on the stack.
+
+/// A single cable: from slot `src` output to slot `dst` input.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct Cable {
+    pub src: usize,
+    pub dst: usize,
+}
+
+/// A fixed-capacity patch with up to `MAX_CABLES` connections.
+pub struct Patch<const MAX_CABLES: usize> {
+    cables: heapless::Vec<Cable, MAX_CABLES>,
+}
+
+impl<const MAX_CABLES: usize> Patch<MAX_CABLES> {
+    pub fn new() -> Self {
+        Self { cables: heapless::Vec::new() }
+    }
+
+    /// Add a cable; returns `Err(cable)` if the patch is full.
+    pub fn connect(&mut self, src: usize, dst: usize) -> Result<(), Cable> {
+        self.cables.push(Cable { src, dst }).map_err(|_| Cable { src, dst })
+    }
+
+    pub fn disconnect(&mut self, src: usize, dst: usize) {
+        self.cables.retain(|c| !(c.src == src && c.dst == dst));
+    }
+
+    pub fn cables(&self) -> &[Cable] {
+        &self.cables
+    }
+}
+
+impl<const MAX_CABLES: usize> Default for Patch<MAX_CABLES> {
+    fn default() -> Self { Self::new() }
+}