fix(spline): edit fit-point splines, and stop deleting the kept half

Two faults, reported together.

Fit-point splines could not be edited at all. A SPLINE is stored either as
a control polygon the curve approaches or as fit points it must pass
through with optional end tangents, and `spline_to_nurbs` only read the
first. A fit-point spline has no control points, so every command that
touched one — TRIM, BREAK, LENGTHEN, OFFSET — got nothing back and did
nothing. Two fit points with start and end tangents, which is what a
drawing stores for the simplest curve, was the clearest case. Fit points
now interpolate into an exact curve through cadkernel, so they are as
editable as any other spline.

Trimming deleted the whole spline when the surviving piece reached the
curve's own start. `trim_spline` cut away everything before the interval
and everything after it, and a cut at either end is a no-op that reports
nothing to do — which the end case handled and the start case propagated
as failure, dropping the piece. That is why one side of the cutting line
worked and the other emptied the drawing.

Elevation is read from whichever point list the spline actually has. A
fit-point spline has no control points, so it was falling to z = 0 on the
first edit.

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#33aa850ac2a27fdb2b305a67df27cb0eae23acc2"
+source = "git+https://github.com/HakanSeven12/cadkernel.git#e76334b7697a3c39292c8a6494d9b837bba78a6e"
 dependencies = [
  "cavalier_contours",
 ]