math, access: correct the parallel notes and a test's geometry

The parallelism note linked to a type the module does not import, which
rustdoc rejects, so it reads as plain text now. The access batch test
computed a propagated state it did not use; it now takes the shell radius
from it so the track sits at a realistic altitude.

crates/foundation/apsis-math/src/parallel.rs

@@ -5,7 +5,7 @@
 //! from several cores are all embarrassingly parallel loops over independent
 //! items, and the standard library's scoped threads express those directly.
 //! `std::thread::scope` borrows its data, so a caller can share a slice or a
-//! reader without an [`Arc`], and joins every thread before returning, so no
+//! reader without an atomic reference count, and joins every thread before returning, so no
 //! result escapes the scope still being written.
 //!
 //! The entry point is [`for_each_index`], which splits a range into one chunk

crates/network/apsis-access/tests/batch.rs

@@ -34,13 +34,17 @@ fn track(satellite: &Satellite, samples: usize) -> Vec<OrbitSample> {
     (0..samples)
         .map(|index| {
             let seconds = index as f64 * 60.0;
-            let state = satellite
+            // The propagated radius sets the shell the synthetic longitude
+            // sweep runs on, so the track is at a realistic altitude.
+            let radius = satellite
                 .propagate_minutes(seconds / 60.0)
-                .expect("the track propagates");
-            // A slow sweep in longitude with a constant latitude stands in
-            // for the rotation into the Earth-fixed frame.
+                .expect("the track propagates")
+                .position
+                .norm();
+            // A slow sweep in longitude stands in for the rotation into the
+            // Earth-fixed frame; the exact geometry is not what this test is
+            // about, only that the batch search reproduces the sequential one.
             let angle = seconds / 4_000.0;
-            let radius = 6_900.0;
             let position = Vec3::new(radius * angle.cos(), radius * angle.sin(), radius * 0.3);
             OrbitSample::new(seconds, position)
         })