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/modules/draw/clipboard/copy_base.rs

@@ -1,6 +1,6 @@
 // COPYBASE — copy the current selection to the clipboard using a base point the
-// user picks (rather than the selection's centroid, as COPYCLIP does). The
-// command only collects the base point; the host performs the copy when it
+// user picks (rather than the selection's lower-left corner, as COPYCLIP does).
+// The command only collects the base point; the host performs the copy when it
 // receives the dispatched `COPYBASE_AT <x> <y> <z>` token.
 
 use glam::DVec3;