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

.claude/settings.json

@@ -0,0 +1,11 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(cargo build:*)",
+      "Bash(grep -v \"^$\")",
+      "WebFetch(domain:github.com)",
+      "Bash(find /Users/mattsp/.cargo/registry/src -name memory.x -path */rp*)",
+      "Bash(find /Users/mattsp/.cargo/registry/src -name build.rs -path */rp-pac*)"
+    ]
+  }
+}