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/.cargo/config.toml

@@ -0,0 +1,12 @@
+[build]
+target = "thumbv8m.main-none-eabihf"   # Cortex-M33 (RP2350)
+
+[target.thumbv8m.main-none-eabihf]
+rustflags = [
+    "-C", "link-arg=-Tdefmt.x",    # defmt-rtt section placement
+    "-C", "link-arg=-Tlink.x",     # cortex-m-rt startup/vector table (includes memory.x)
+]
+runner = "probe-rs run --chip RP2350"
+
+[env]
+DEFMT_LOG = "debug"