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-embedded/src/main.rs

@@ -0,0 +1,66 @@
+//! synth-embedded — RP2350 / Embassy audio engine entry point.
+//!
+//! Hardware assumed (adjust pin numbers in spawner calls to match your wiring):
+//!   GPIO  9 → I2S BCK  (bit clock)
+//!   GPIO 10 → I2S LRCK (word select / left-right clock)
+//!   GPIO 11 → I2S DATA
+//!   GPIO  1 → UART0 RX (MIDI in, 31250 baud)
+//!
+//! Build:
+//!   cargo build --release
+//!
+//! Flash (requires probe-rs and a debug probe):
+//!   cargo run --release
+
+#![no_std]
+#![no_main]
+
+use defmt_rtt as _;
+use panic_probe as _;
+
+use embassy_executor::Spawner;
+use embassy_rp::{
+    bind_interrupts, dma,
+    peripherals::{DMA_CH0, DMA_CH1, PIO0, UART0},
+    pio::InterruptHandler as PioIrqHandler,
+    uart::InterruptHandler as UartIrqHandler,
+};
+use embassy_sync::{blocking_mutex::raw::ThreadModeRawMutex, mutex::Mutex};
+
+mod audio;
+mod midi;
+mod params;
+
+use params::SynthParams;
+
+/// Shared synthesiser state — written by the MIDI task, read by the audio task.
+///
+/// `ThreadModeRawMutex` is appropriate here: both tasks run on core 0 and
+/// the mutex is never accessed from an interrupt handler.
+pub static PARAMS: Mutex<ThreadModeRawMutex, SynthParams> = Mutex::new(SynthParams::new());
+
+// Register Embassy interrupt handlers for the peripherals we use.
+bind_interrupts!(struct Irqs {
+    PIO0_IRQ_0 => PioIrqHandler<PIO0>;
+    UART0_IRQ  => UartIrqHandler<UART0>;
+    DMA_IRQ_0  => dma::InterruptHandler<DMA_CH0>, dma::InterruptHandler<DMA_CH1>;
+});
+
+#[embassy_executor::main]
+async fn main(spawner: Spawner) {
+    let p = embassy_rp::init(Default::default());
+
+    defmt::info!("synth-embedded starting on RP2350");
+
+    // Audio output via PIO0 + DMA (I2S to PCM5102A or compatible DAC).
+    spawner.spawn(audio::audio_task(
+        p.PIO0,
+        p.PIN_9,   // BCK
+        p.PIN_10,  // LRCK
+        p.PIN_11,  // DATA
+        p.DMA_CH0,
+    ).unwrap());
+
+    // MIDI input via UART0 RX.
+    spawner.spawn(midi::midi_task(p.UART0, p.PIN_1, p.DMA_CH1).unwrap());
+}