kernel: pcurve fits start from four samples per span

Each sample of an intersection curve is one of truck's double projections
(about 0.13 ms in a release build), and a fit evaluates the curve at every
sample and halfway between. Starting from four samples a span instead of
eight halves that; the checks stay at eight points a span, and the knots
at the curve's own breaks already carry the shape. The cylinder cut by a
tilted box now takes 70 ms at 1e-3 mm and 149 ms at 1e-5 mm for all its
faces (release), from 183 and 638 ms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QWhMwdqJYQKQq8RCLSL1LT

crates/kernel/src/native/pcurve.rs

@@ -60,7 +60,7 @@ const MAX_SPANS: usize = 4000;
 /// Refinement rounds of one fit.
 const MAX_ROUNDS: usize = 40;
 /// Samples per knot span when a fit starts.
-const PER_SPAN: usize = 8;
+const PER_SPAN: usize = 4;
 /// Spans a fit starts from (more where the curve has its own knots).
 const START_SPANS: usize = 4;