refactor(modify): take plane geometry from cadkernel

TRIM, FILLET and EXPLODE each carried their own copy of the same plane
maths. The copies had already drifted apart: `ll` differed only in
parameter names between trim.rs and fillet.rs, `arc_pts` only in which
angle helper it called, and `norm` meant angle normalisation in trim.rs
and explode.rs while meaning vector normalisation in two other files.

The implementations now live in cadkernel's geom2d. A new local `geom`
module adapts the call shapes — the kernel takes `[f64; 2]` points and
returns `[f64; 3]`, while these commands pass loose scalars and feed
`WireModel::points`, which is `[f32; 3]`. Narrowing stays on this side
because only the caller knows whether it is a preview wire, which leaves
`points_low` empty and accepts the loss, or resident geometry, which
splits the f64 into a high/low pair instead.

Call sites are untouched: the kernel functions are imported under the old
local names, so the signatures they were written against still hold.

This also fixes ELLIPSE trim and extend previews. `ellipse_pts` wrote its
components as [x, z, y] while `arc_pts` — reached from the same match in
`entity_pts`, feeding the same `[f32; 3]` — wrote [x, y, z], and
`WireModel::points` is documented as world-space positions. Ellipse
previews were collapsing onto y = 0 and displacing along Z. The kernel's
`ellipse_arc` orders them correctly, so delegating corrects it.

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

src/modules/draw/modify/mod.rs

@@ -8,6 +8,7 @@ pub mod delete;
 mod entity_index;
 pub mod explode;
 pub mod fillet;
+pub mod geom;
 pub mod join;
 pub mod lengthen;
 pub mod mirror;