feat(entities): give an entity its geometry as a curve

The same shape was being read out of an entity's fields in three places
and the three had drifted. An arc's tessellation cut it at 20 segments
per radian, TRIM's sampling at 16, and its snap points came from a
fourth expression again; a spline was sampled at a fixed 96 points
however long it was; TRIM read an arc's centre as world coordinates
when the entity stores it in its OCS, so an arc carrying the (0, 0, −1)
normal a MIRROR leaves behind acted as a boundary on the wrong side of
the drawing.

entity_curve returns a PlanarCurve: the shape, plus the plane its
coordinates are read in. The plane comes from the entity's OCS — its
extrusion normal through the DXF arbitrary-axis algorithm — not from the
UCS. A UCS is the working plane a user drew in and an entity does not
remember it: draw a circle in a UCS turned 30° about Z and the stored
normal is still +Z, which is why the header keeps the UCS separately.
The algorithm itself stays in transform.rs, where the format is
understood and where #142's degenerate case was fixed once.

Entities that store world coordinates and may not be planar at all —
LINE, SPLINE — are checked rather than assumed. A spline whose points
wander in space returns None instead of being flattened onto XY, which
would move it.

entity_curve_xy folds the plane away for the editing commands, which
work in plan view and need every boundary in one frame before two of
them can be intersected.

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#dee251f190a82f66d548e10d36f112815d9a901f"
+source = "git+https://github.com/HakanSeven12/cadkernel.git#39083307452ba14b3968bdb0738e2665d3d18e62"
 dependencies = [
  "cavalier_contours",
 ]