fix(print): send Windows plots to the chosen printer, name the default, report failures

On Windows a plot with no printer chosen went to the PDF application's
bare "print" verb — whatever that application considered its default —
and a viewer without a "printto" verb (Edge, store readers) silently
dropped the printer the dialog had chosen. The dialog also never named the
system default, and a failed printer enumeration left only "Default" and
"PDF" with the reason written to a console nobody sees.

- The default printer is looked up by name (GetDefaultPrinterW) both for
  the dialog's "System default printer (…)" entry and before every job.
- A pure route decision (plan_print_route) resolves "Default" to that
  name and uses the PDF application only when it registered a "printto"
  verb for .pdf (AssocQueryStringW); otherwise the job goes straight to
  the spooler through the existing GDI path. The report says which:
  "Sent to printer: NAME (direct print | via the PDF application)".
- list_printers reports the system's own error; the plot dialog shows it
  under the printer row and the command line warns once. A desktop without
  CUPS is an empty list, not an error.
- PRINTERS command: default printer, listed printers (or the error) and,
  on Windows, the print route — a report to paste into an issue.
- Properties… on the Default entry opens that printer's preferences sheet.
- GDI path: the driver's dmSize is kept (its private data lives behind
  it), the print raster is no longer memoised for the session, and a
  spooled temporary PDF is removed.

Tests: route planning (named / blank / no default / printto or not), the
PRINTERS command; the two Windows-only GDI tests pass the resolved name.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/io/plot_device.rs

@@ -232,8 +232,14 @@ pub fn default_printer_name() -> Option<String> {
     (!name.is_empty()).then(|| name.to_string())
 }
 
-/// Windows default printer discovery lands with native GDI printing.
-#[cfg(any(target_arch = "wasm32", target_os = "windows"))]
+/// The spooler's default printer, by name — the same lookup every direct
+/// print job resolves "Default" through, so the dialog names what will print.
+#[cfg(target_os = "windows")]
+pub fn default_printer_name() -> Option<String> {
+    crate::io::print_to_printer::windows_default_printer().ok()
+}
+
+#[cfg(target_arch = "wasm32")]
 pub fn default_printer_name() -> Option<String> {
     None
 }