Scaffold workspace, shared types and requirements spec

Cargo workspace with core/ble/fit/probe crates. crates/core/src/types.rs
is the fixed contract between the BLE layer, ride engine and UI.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,20 @@
+# Rust
+/target
+**/*.rs.bk
+
+# Node
+node_modules/
+dist/
+.vite/
+
+# Tauri
+src-tauri/target/
+src-tauri/gen/
+
+# Editors / OS
+.DS_Store
+*.swp
+
+# Ride data
+*.fit
+/rides/