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 <noreply@anthropic.com>

src/modules/draw/modify/geom.rs

@@ -51,11 +51,6 @@ pub fn line_circle(px: f64, py: f64, dx: f64, dy: f64, cx: f64, cy: f64, r: f64)
     geom2d::line_circle([px, py], [dx, dy], [cx, cy], r)
 }
 
-/// Angles **on the first circle** where two circles meet.
-pub fn circle_circle_angles(cx1: f64, cy1: f64, r1: f64, cx2: f64, cy2: f64, r2: f64) -> Vec<f64> {
-    geom2d::circle_circle_angles([cx1, cy1], r1, [cx2, cy2], r2)
-}
-
 /// `(s_on_line, t_on_ellipse)` pairs where a line meets an ellipse.
 ///
 /// `nx, ny` is the unit major axis.