refactor(curve): draw a fit spline and an offset preview from the curve

Two places were left describing geometry for themselves.

A fit-point SPLINE was drawn from a C² cubic solved in spline.rs while
TRIM and snap cut a separate interpolation from the kernel. Near enough
to look right, far enough apart that a trim landed beside the line it
was aimed at. It now draws the curve those commands work on, and falls
back to the local solve only where the kernel declines — a fit spline
through points in space is not a planar curve, so there is nothing for
it to say.

Closed fit splines were worse than that: the kernel's interpolation is a
clamped solve with no wrap in it, so the curve TRIM used never returned
to its start and the seam cut nothing. Repeating the first fit point
closes it. The seam is C¹ where the rest of the curve is C², which is
the honest limit of a clamped solve — the alternative was a curve with a
gap in it.

OFFSET's preview sampling had drifted the same way TRIM's had: an arc
and a circle read `center.x/.y` as world coordinates when the entity
stores them in its OCS, so an extruded one previewed on the wrong side;
every curved kind was cut at a fixed twenty segments per radian whatever
its size; and an ellipse rebuilt its axis frame from the major axis' X
and Y alone, dropping the Z the entity stores. A hundred lines of that
become one tessellation. A RAY now previews as far as an XLINE does
rather than as a stub the length of its direction vector.

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

src/modules/draw/modify/spline_ops.rs

@@ -78,7 +78,20 @@ fn spline_to_nurbs_with(
     }
 
     // No usable control polygon, so this is a fit-point spline.
-    let fit: Vec<[f64; 2]> = spl.fit_points.iter().map(&point).collect();
+    let mut fit: Vec<[f64; 2]> = spl.fit_points.iter().map(&point).collect();
+    if spl.flags.closed || spl.flags.periodic {
+        // The interpolation is a clamped solve and does not model a wrap, so
+        // a closed spline came back as an open curve that never returned to
+        // its start — and a TRIM against it then cut nothing along the seam.
+        // Repeating the first point closes it. The seam is C¹ rather than the
+        // C² the rest of the curve has, which is the honest limit of a
+        // clamped solve; the alternative was a curve with a gap in it.
+        if let (Some(&first), Some(&last)) = (fit.first(), fit.last()) {
+            if first != last {
+                fit.push(first);
+            }
+        }
+    }
     let tangent = |v: &Vector3| {
         let flat = vector(v);
         (flat[0] * flat[0] + flat[1] * flat[1] > 1e-18).then_some(flat)