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 <[email protected]>

src/locale_catalog.rs

@@ -2885,6 +2885,10 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "SLICE: a result could not be committed; all source objects were retained." => Some(("common", "slice-result-commit-failed")),
         "SLICE: a retained result could not be updated." => Some(("common", "slice-retained-update-failed")),
         "SLICE: created %{count} result object(s)." => Some(("common", "slice-created-results")),
+        "SECTIONCULL: a cut object could not be prepared; nothing was changed." => Some(("common", "sectioncull-prepare-failed")),
+        "SECTIONCULL: erased %{erased} object(s) and cut %{cut} across the section." => Some(("common", "sectioncull-result")),
+        "SECTIONCULL: nothing lies outside the section." => Some(("common", "sectioncull-nothing-outside")),
+        "SECTIONCULL: select a section object or turn on a live section." => Some(("common", "sectioncull-no-section")),
         "SOLID  Specify first point:" => Some(("draw", "solid-specify-first-point")),
         "SOLID  Specify fourth point:" => Some(("draw", "solid-specify-fourth-point")),
         "SOLID  Specify second point:" => Some(("draw", "solid-specify-second-point")),