Record the measured validation and performance figures in the readme

The acceptance criteria ask for numbers rather than claims, and every row but
the three still-unbuilt ones has been measured. Splits validation into the
Orekit comparison and the analytic invariants, and gives the performance table
step counts alongside times so a ratio of times can be read as per-step cost.

The three invariant tests and the MSIS drag comparison now print what they
achieved, so the readme's figures can be reproduced by running them.

Also corrects the ordinary-diffeq version in the foundations table, and the
claim that the fixture generator produces observations, which it does not yet.

tests/orekit_comparison.rs

@@ -335,11 +335,13 @@ fn test_msis_drag_agrees_with_orekit() {
     .accuracy(Accuracy::High);
 
     let solution = propagation.run();
-    let mut position = 0.0_f64;
+    let (mut position, mut velocity) = (0.0_f64, 0.0_f64);
     for state in &arc.states {
         let ours = Orbit.state(solution.interpolate(state.elapsed));
         position =
             position.max((ours.position() - Vector3::from_column_slice(&state.position)).norm());
+        velocity =
+            velocity.max((ours.velocity() - Vector3::from_column_slice(&state.velocity)).norm());
     }
 
     let drag_free = {
@@ -353,7 +355,7 @@ fn test_msis_drag_agrees_with_orekit() {
     let effect = (Vector3::from_column_slice(&last.position) - drag_free).norm();
 
     println!(
-        "msis drag: {position:.3e} m, {:.4}% of a {effect:.0} m effect",
+        "msis drag: {position:.3e} m, {velocity:.3e} m/s, {:.4}% of a {effect:.0} m effect",
         position / effect * 100.0
     );
     assert!(