refactor(trim): find boundary crossings with cadkernel

`line_seg_ts`, `arc_seg_ts`, `ellipse_seg_ts` and `spline_seg_ts` each
walked every boundary kind and solved that pair by hand — four nested
matches over the same seven cases. They are now four descriptions of the
target as a `Curve` handed to one `cut_params`, which asks the kernel.
Their signatures are unchanged, so the twenty call sites are untouched.

Three things follow from it.

Boundary splines and target splines stop being sampled. A cut on a spline
now lands where the boundary actually crosses rather than where a fixed
sampling said it did, because the kernel subdivides and converges to a
tolerance instead.

Rays and infinite lines as boundaries are handled by the kernel's extent
rather than by a case here — a ray rejecting what lies behind its origin
is the kernel's job now.

`extend_spline` shoots a real ray. It used to build a line a million units
long and cut against that, which both invents a limit and, at survey
coordinates, is comparable to the coordinates themselves. The ray's
direction is a unit vector, so what comes back is the distance to the
boundary. That also fixes the hit position, which multiplied by the
million-unit extent a second time and landed the extended end far past
where it was supposed to.

`cut_params` clamps to 0..=1 only for a bounded target; a ray has to keep
the parameter it comes back with, since the distance is the answer.

Co-Authored-By: Claude Opus 5 <[email protected]>

Cargo.lock

@@ -945,7 +945,7 @@ checksum = "fc652a48c352aef3ea3aed32080501cf3ef6ed5da78602a020c991775b0aff04"
 [[package]]
 name = "cadkernel"
 version = "0.1.0"
-source = "git+https://github.com/HakanSeven12/cadkernel.git#e76334b7697a3c39292c8a6494d9b837bba78a6e"
+source = "git+https://github.com/HakanSeven12/cadkernel.git#5294a45d19508c22726c01b91215d3ca3fd4c815"
 dependencies = [
  "cavalier_contours",
 ]