fix(spline): reverse a spline without reshaping it

SPLINEDIT's REVERSE reversed the control points and then regenerated a
clamped *uniform* knot vector. That is only harmless for a spline whose
knots were evenly spaced to begin with; for any other — which is most of
them once a modeller has been near one — the curve came back a different
shape, bulging where it had been taut. It now mirrors the knots within
the domain, which is exact, and swaps the end tangents to match.

PEDIT's Spline option evaluated a uniform cubic B-spline from four
blending polynomials written out by hand, clamped the open case by
repeating end control points, and then pinned the two endpoints
afterwards to undo the drift that leaves. A clamped knot vector says all
of that directly, and the closed case gets a genuinely periodic curve
rather than one whose seam is a special case.

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

Cargo.lock

@@ -935,7 +935,7 @@ checksum = "fc652a48c352aef3ea3aed32080501cf3ef6ed5da78602a020c991775b0aff04"
 [[package]]
 name = "cadkernel"
 version = "0.1.0"
-source = "git+https://github.com/HakanSeven12/cadkernel.git#69a8bf2b747c8b0a4abe6a571859c4c3a4f610d2"
+source = "git+https://github.com/HakanSeven12/cadkernel.git#5499242f1160018e066f78d4e462c9532e665a96"
 dependencies = [
  "cavalier_contours",
 ]