fix(select): accumulate selection set, apply on Enter/RMB (#220)

Modify commands invoked with nothing pre-selected (ERASE, MOVE, COPY,
ROTATE, SCALE, MIRROR, ARRAY*, STRETCH, EXPLODE, GROUP, BLOCK, the
clipboard cuts, LAYOFF/FRZ/LCK/ULK, ...) ran the command on the FIRST
picked object and terminated. You could not build up a selection.

SelectObjectsCommand now keeps gathering: each pick accumulates into the
set (Shift removes), the prompt shows the running count, and the command
fires only when the user presses Enter or right-clicks. Reuses the
existing JOIN gather pattern, so no viewport/host changes are needed.

LAYMCUR acts on a single object's layer, so it keeps the immediate
first-pick behaviour via the new SelectObjectsCommand::instant().

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/commands/layers.rs

@@ -301,7 +301,8 @@ impl OpenCADStudio {
                 let entities = self.tabs[i].scene.selected_entities();
                 if entities.is_empty() {
                     use crate::modules::draw::select::SelectObjectsCommand;
-                    let cmd = SelectObjectsCommand::new("LAYMCUR");
+                    // LAYMCUR acts on one object's layer; apply on the first pick.
+                    let cmd = SelectObjectsCommand::instant("LAYMCUR");
                     self.command_line.push_info(&cmd.prompt());
                     self.tabs[i].active_cmd = Some(Box::new(cmd));
                 } else {