feat(clipboard): paste menu with paste-to-original and paste-as-block

Turn the ribbon Clipboard "Paste" button into a dropdown:
- Paste — existing pick-a-point paste.
- Paste to Original Coordinates — drops the clipboard at its source
  coordinates with no prompt.
- Paste as Block — wraps the clipboard contents in a new block definition
  and starts an interactive insert (prompts for the drop point, with the
  geometry rubber-banding under the cursor).

Also fixes two bugs the new commands exposed:
- entities_centroid summed the NaN separator points wire models carry,
  so the paste base point came out NaN; non-finite points are now
  skipped. A NaN base previously froze block tessellation outright.
- PASTEORIG added clipboard entities with their original handles
  (duplicates on same-document paste); it now goes through
  add_entity_clone and merges referenced records like the other pastes.

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

src/app/cmd_result.rs

@@ -1682,7 +1682,7 @@ impl OpenCADStudio {
     /// this drawing doesn't already have. Each recreated record gets a fresh
     /// handle from the target document so it can't collide with an existing
     /// one. No-op for same-document pastes (the records already exist). (#129)
-    fn merge_clipboard_deps(&mut self, i: usize) {
+    pub(super) fn merge_clipboard_deps(&mut self, i: usize) {
         use acadrust::TableEntry;
         if self.clipboard_deps.is_empty() {
             return;