fix(clipboard): paste anchor at bounding-box lower-left, not center (#285)

COPYCLIP/CUTCLIP used the mean of each entity's bounding-box centre as the
paste anchor. Standard clipboard behaviour puts the lower-left corner of the
box enclosing all copied items under the cursor. Union each entity's bbox min
corner instead, and rename clipboard_centroid -> clipboard_base (COPYBASE also
stores a picked point there, so "centroid" was inaccurate).

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

src/app/command_driver.rs

@@ -790,7 +790,7 @@ impl OpenCADStudio {
                 if self.clipboard.is_empty() {
                     self.command_line.push_error("Clipboard is empty.");
                 } else {
-                    let delta = base_pt - self.clipboard_centroid;
+                    let delta = base_pt - self.clipboard_base;
                     let translate = crate::command::EntityTransform::Translate(delta);
                     self.push_undo_snapshot(i, "PASTECLIP");
                     let count = self.clipboard.len();