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

Cargo.toml

@@ -0,0 +1,60 @@
+[workspace]
+resolver = "2"
+members = [
+    "crates/synth-core",
+    "crates/synth-visualiser",
+    "crates/synth-embedded",
+]
+
+[profile.release]
+opt-level = 3
+lto = "thin"
+codegen-units = 1
+strip = "symbols"
+
+[profile.release-wasm]
+inherits = "release"
+opt-level = "z"
+lto = "fat"
+panic = "abort"
+
+[workspace.dependencies]
+# no_std-compatible math
+libm        = { version = "0.2",  default-features = false }
+num-traits  = { version = "0.2",  default-features = false }
+micromath   = { version = "2.1",  default-features = false }
+
+# Stack-allocated collections
+heapless    = { version = "0.8",  default-features = false }
+
+# MIDI
+midi-types  = { version = "0.1",  default-features = false }
+
+# WASM / browser
+wasm-bindgen         = { version = "0.2" }
+js-sys               = { version = "0.3" }
+wasm-bindgen-futures = { version = "0.4" }
+console_error_panic_hook = { version = "0.1" }
+
+# Serialisation
+serde = { version = "1", default-features = false, features = ["derive"] }
+
+# Embassy — RP2350 support is only in the git repo, not crates.io.
+# Cargo pins the resolved commit into Cargo.lock on first build.
+# Pin a specific `rev = "..."` for reproducible builds.
+embassy-executor = { git = "https://github.com/embassy-rs/embassy", default-features = false }
+embassy-rp       = { git = "https://github.com/embassy-rs/embassy", default-features = false }
+embassy-sync     = { git = "https://github.com/embassy-rs/embassy", default-features = false }
+embassy-time     = { git = "https://github.com/embassy-rs/embassy", default-features = false }
+embassy-futures  = { git = "https://github.com/embassy-rs/embassy", default-features = false }
+
+# Embedded ecosystem (crates.io — stable)
+cortex-m     = { version = "0.7", default-features = false }
+cortex-m-rt  = { version = "0.7", default-features = false }
+defmt        = { version = "1.0.1", default-features = false }
+defmt-rtt    = { version = "1.1.0", default-features = false }
+panic-probe  = { version = "1.0.0", default-features = false }
+fixed        = { version = "1.23", default-features = false }
+static-cell  = { version = "2",   default-features = false }
+pio          = { version = "0.3", default-features = false }
+# pio-proc     = { version = "0.3" }