feat(model): SECTIONCULL erases what lies outside a section

SECTIONCULL keeps only what the selected section object (or the live
section of the space) shows: solids, surfaces and closed meshes are cut
to it with the same planes the live section uses, open meshes are
clipped face by face, and every other object wholly outside it is
erased. Objects across the boundary that cannot be cut stay whole. One
undo step.

The section's bounding planes are gathered once (section_planes) and
shared by the live section and the cull.

Refs #1407

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/commands/draw.rs

@@ -2148,6 +2148,11 @@ impl OpenCADStudio {
                 self.tabs[i].active_cmd = Some(Box::new(subtract));
             }
 
+            // SECTIONCULL — erase what lies outside a section object.
+            "SECTIONCULL" => {
+                return Some(self.section_cull());
+            }
+
             // INTERFERE — non-destructive intersect: solid from the overlap.
             "INTERFERE" => {
                 return Some(self.solid_interfere());

src/app/commands/mod.rs

@@ -621,6 +621,7 @@ inventory::submit!(crate::command::CommandRegistration {
         "SL",
         "INTERFERE",
         "INF",
+        "SECTIONCULL",
         "PRESSPULL",
         "THICKEN",
         "3DROTATE",