feat(modify): EXTRIM — cookie-cutter trim on one side of a boundary (#253)

Run EXTRIM, pick a cutting boundary (line/arc/circle/ellipse/spline/
polyline), then click a side: every object crossing the boundary is
trimmed on that side and objects wholly on it are erased. Side is decided
by a parity crossing count (a segment to the pick point crossing the
boundary an even number of times is on the pick side), reusing the trim
intersection machinery; line boundaries extend to infinity. Line/arc/
circle are trimmed exactly; polyline/ellipse/spline via dense sampling.

Live preview while choosing the side: boundary in yellow, surviving parts
in blue, the removed parts in red.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/commands/mod.rs

@@ -462,6 +462,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "EXPORT",
         "EXPORTSTEP",
         "EXPORTSTL",
+        "EXTRIM",
         "FILETAB",
         "FIND",
         "FLATTEN",