feat: PRINT command — send layout to system printer via lp/lpr

The PRINT command renders the current layout to a temporary PDF (reusing
the existing PDF export pipeline) and dispatches it to the OS default
printer:
  - Linux/macOS: `lp` (CUPS) with `lpr` as fallback.
  - Windows: ShellExecute "print" verb (compiled-in, unused on Linux).

PLOT and EXPORT still open the save-file dialog as before.
Status messages are shown in the command line during the async job.

Closes ROADMAP 1.8.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/io/mod.rs

@@ -6,6 +6,7 @@
 pub mod obj;
 pub mod pdf_export;
 pub mod plot_style;
+pub mod print_to_printer;
 pub mod stl;
 pub mod xref;