Gearing as development, physics-derived load model, 105kg rider

Gears are metres per crank revolution rather than gradient offsets, and
resistive_force_n exposes what the road is doing at a given speed so load
can be computed directly instead of servoed.

The load model is tested but not yet commanded: FTMS sim mode has the
trainer compute rolling and aero itself, so sending a gradient that already
contains them would double-count. Needs Crr/Cw zeroed and a ride to verify.

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

crates/core/src/physics.rs

@@ -250,6 +250,27 @@ pub fn equilibrium_speed_mps(power_w: f32, gradient_pct: f32, cfg: &RiderConfig)
     0.5 * (lo + hi)
 }
 
+/// Total resistive force at a given speed and gradient, newtons.
+///
+/// This is what the road is doing to the rider: gravity down the slope, rolling
+/// resistance, and aerodynamic drag rising with the square of speed. It is the
+/// force a trainer must reproduce at the wheel for the ride to feel real, and
+/// therefore the basis for virtual gearing (see `crate::gearing`).
+pub fn resistive_force_n(speed_mps: f32, gradient_pct: f32, cfg: &RiderConfig) -> f32 {
+    let forces = Forces::new(0.0, gradient_pct, cfg);
+    let v = if speed_mps.is_finite() {
+        speed_mps.clamp(0.0, MAX_SPEED_MPS)
+    } else {
+        0.0
+    };
+    let f = forces.resistive_n + forces.drag_k * v * v;
+    if f.is_finite() {
+        f
+    } else {
+        0.0
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;