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/src/physics.rs

@@ -0,0 +1,63 @@
+//! Virtual speed from measured power (§5.7 of REQUIREMENTS.md).
+//!
+//! The app owns the physics rather than trusting the trainer's reported speed
+//! (FR-7.1). This makes ride behaviour reproducible in tests, independent of
+//! the trainer's internal mass assumptions, and is a prerequisite for virtual
+//! gearing later.
+//!
+//! Per tick:
+//! ```text
+//! F_propulsive = (P × drivetrain_efficiency) / max(v, v_min)
+//! F_gravity    = m × g × sin(atan(gradient))
+//! F_rolling    = m × g × Crr × cos(atan(gradient))
+//! F_aero       = ½ × ρ × CdA × v²
+//! a            = (F_propulsive − F_gravity − F_rolling − F_aero) / m
+//! v           += a × Δt      (clamped at ≥ 0)
+//! ```
+
+use crate::types::RiderConfig;
+
+pub const GRAVITY: f32 = 9.80665;
+
+/// Speed floor used to keep `P / v` finite at a standstill. Also the speed
+/// below which the rider is considered stopped.
+pub const MIN_SPEED_MPS: f32 = 0.5;
+
+/// Evolving physical state of the virtual rider.
+#[derive(Debug, Clone, Copy, PartialEq, Default)]
+pub struct PhysicsState {
+    /// Virtual speed, metres per second.
+    pub speed_mps: f32,
+    /// Virtual distance travelled, metres.
+    pub distance_m: f64,
+    /// Cumulative elevation gained, metres.
+    pub elevation_gain_m: f32,
+}
+
+impl PhysicsState {
+    /// Advance the simulation by `dt` seconds under `power_w` at `gradient_pct`.
+    ///
+    /// Must model inertia (FR-7.3) — speed accelerates toward equilibrium
+    /// rather than snapping to it — and must never produce negative speed,
+    /// NaN, or unbounded values for any finite input.
+    pub fn step(&mut self, power_w: f32, gradient_pct: f32, cfg: &RiderConfig, dt: f32) {
+        let _ = (power_w, gradient_pct, cfg, dt);
+        todo!("implemented in crates/core/src/physics.rs — see AGENT task A")
+    }
+
+    pub fn speed_kph(&self) -> f32 {
+        self.speed_mps * 3.6
+    }
+
+    pub fn is_moving(&self) -> bool {
+        self.speed_mps > MIN_SPEED_MPS
+    }
+}
+
+/// Steady-state speed for a given power and gradient — the speed at which
+/// propulsive and resistive forces balance. Useful for tests and for sanity
+/// checks on the resistance curve later.
+pub fn equilibrium_speed_mps(power_w: f32, gradient_pct: f32, cfg: &RiderConfig) -> f32 {
+    let _ = (power_w, gradient_pct, cfg);
+    todo!("implemented in crates/core/src/physics.rs — see AGENT task A")
+}