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>

crates/core/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "bikecontrol-core"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+serde = { workspace = true }
+serde_yaml_ng = { workspace = true }
+thiserror = { workspace = true }
+roxmltree = { workspace = true }
+
+[dev-dependencies]
+approx = "0.5"