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.toml

@@ -0,0 +1,44 @@
+[package]
+name        = "synth-embedded"
+version     = "0.1.0"
+edition     = "2021"
+description = "Embassy/RP2350 audio engine — I2S out, MIDI in, synth-core DSP"
+publish     = false
+
+[[bin]]
+name = "synth-embedded"
+path = "src/main.rs"
+
+[dependencies]
+synth-core = { path = "../synth-core" }
+
+embassy-executor = { workspace = true, features = [
+    "platform-cortex-m",
+    "executor-thread",
+    "defmt",
+] }
+embassy-rp = { workspace = true, features = [
+    "rp235xa",   # Pico 2 (30 GPIOs); use rp235xb for the 48-pin variant
+    "rt",        # enables rp-pac/rt → generates device.x interrupt vectors
+    "time-driver",
+    "defmt",
+    "unstable-pac",
+] }
+embassy-sync    = { workspace = true, features = ["defmt"] }
+embassy-time    = { workspace = true, features = ["defmt", "defmt-timestamp-uptime"] }
+embassy-futures = { workspace = true }
+
+cortex-m    = { workspace = true, features = ["inline-asm", "critical-section-single-core"] }
+cortex-m-rt = { workspace = true, features = ["device"] }
+defmt       = { workspace = true }
+defmt-rtt   = { workspace = true }
+panic-probe = { workspace = true, features = ["print-defmt"] }
+
+fixed       = { workspace = true }
+heapless    = { workspace = true }
+libm        = { workspace = true }
+pio         = { workspace = true }
+# pio-proc    = { workspace = true }
+
+# Profile overrides must live in the workspace root Cargo.toml, not here.
+# See /Cargo.toml for release/dev profile settings.