feat(plot): QUICKPRINT/QP — plot a selection's bounding box to PDF (#325)

Run QUICKPRINT (or QP), select objects, then Enter: the selection's
bounding box becomes the plot window and a PDF is written next to the
drawing (<name>_<timestamp>.pdf) using the active page setup, with no
dialog. Model space.

Implemented as a selection-gathering CadCommand (src/modules/view/
quick_print.rs) returning CmdResult::QuickPrint(handles); the host
(on_quick_print_handles) unions the picked entities' AABBs, sets the
plot window, and reuses the tested clipped window-export path.

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

src/app/commands/mod.rs

@@ -512,6 +512,8 @@ inventory::submit!(crate::command::CommandRegistration {
         "PROPS",
         "PSPACE",
         "PURGE",
+        "QP",
+        "QUICKPRINT",
         "QS",
         "QSAVE",
         "QSELECT",