refactor(fillet): take the corner arc from cadkernel

`compute_fillet` keeps the entity work — resolving which side of each line
the pick kept, and trimming the lines back — and hands the geometry out.
The kernel takes the corner as an apex plus the two keep directions, which
is what the side resolution here already produces, and returns the tangent
points, centre and the endpoints in fill-the-corner order.

`line_circle_ts` and `circle_circle_pts` were further copies of maths the
kernel already had; the second now calls `circle_circle_points`, added for
callers that want the crossings as points rather than as angles to cut at.

`arc_span` and `clamp_angle_to_arc` stay put on purpose. This file's
`arc_span` treats any span under 1e-6 as a full turn, where the kernel's
only does so at zero or below. That difference is real for a degenerate
arc, and picking a winner is a separate question from moving the code.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.lock

@@ -945,7 +945,7 @@ checksum = "fc652a48c352aef3ea3aed32080501cf3ef6ed5da78602a020c991775b0aff04"
 [[package]]
 name = "cadkernel"
 version = "0.1.0"
-source = "git+https://github.com/HakanSeven12/cadkernel.git#9c7b2b75ab76806d086f2e7eca17750771150dba"
+source = "git+https://github.com/HakanSeven12/cadkernel.git#feed535a0f72f9479d618a4999dba9bafcea250f"
 dependencies = [
  "cavalier_contours",
 ]