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/app/mod.rs

@@ -402,6 +402,10 @@ pub enum Message {
     PlotExport,
     /// Callback after the user picks (or cancels) the export path.
     PlotExportPath(Option<std::path::PathBuf>),
+    /// Send current layout to the system printer (via lp / lpr).
+    PrintToPrinter,
+    /// Callback from the async printer job.
+    PrintResult(Result<String, String>),
     // ── Plot Style Table ─────────────────────────────────────────────────
     /// Open file dialog to load a CTB/STB plot style table.
     PlotStyleLoad,