fix: harden the issue-fix wave after full adversarial review

Five reviewers (PDF, snap, commands, otrack/help, printing) covered the
eight issue-fix commits; every blocker and major is fixed here.

Printing (#1261 follow-up):
- printto params now quote the printer name and pad the empty driver/
  port slots handlers substitute ("Microsoft Print to PDF" was split
  into three arguments and the job silently lost).
- Level-2 enumeration also merges when the level-4 list omits the
  configured default, not only when it is empty; a level-2 error no
  longer discards the default-printer merge.
- The printer list loads in a background task instead of blocking the
  UI thread (level-2 queries can wait out RPC timeouts).
- CTB registration: collisions keep the existing file but now say so,
  and only container-parsed tables are registered (the permissive
  zlib heuristic stays session-only so junk cannot persist).

Snapping (#1030/#1319 follow-up):
- Edge-wire key_vertices gain NaN separators: junctions between
  unrelated mesh edges no longer fabricate phantom midpoints.
- The wire cap counts segments (not separator points) and decimates
  instead of dropping, so dense meshes keep partial endpoint coverage
  instead of losing snap entirely.
- f64 endpoint recovery is a one-shot per-wire map instead of an
  O(points x geoms) rescan on every mouse move.

Commands (#1082 class): the same typed-value-on-web bug also hit
CYLINDER/BOX/WEDGE/CONE/PYRAMID heights, HELIX final height (wrong
value, not just dropped), and SWEEP reference/new lengths — all now
commit as text at exactly those steps, each pinned by a regression
test that fails without the override (plus the missing native-route
twin for EXTRUDE).

PDF (#702 follow-up): heavy CTB pens bypass the Form XObject (their
half-width exceeds the /BBox SDF margin and was flat-chopped), CTM
scale entries round to 4 significant digits (0.01 quantized small
text by ~1%), degenerate quads skip emission, the atlas snapshot is
taken once per export instead of per text entity, and the rounding
comments no longer conflate page points with glyph-plane units.

OTRACK (#1313 follow-up): the guide's polar quantization diverges
from the snapper's drafting-gated polar during window-corner drags —
documented; an orphaned grid-step doc block is reunited with its
function.

src/app/update/mod.rs

@@ -8335,12 +8335,19 @@ impl OpenCADStudio {
                     .push_error(crate::tf!("Print failed: {e}").as_ref());
                 Task::none()
             }
+            // The printer list arrives after the Plot dialog already opened
+            // (enumeration runs off the UI thread); assigning it re-renders
+            // the dropdown while leaving the user's selection untouched.
+            Message::PrintersListed(printers) => {
+                self.plot_dialog.printers = printers;
+                Task::none()
+            }
 
             // ── Plot Style Table ──────────────────────────────────────────────
             Message::PlotStyleLoad => {
                 Task::perform(crate::io::pick_plot_style(), Message::PlotStyleLoaded)
             }
-            Message::PlotStyleLoaded(Ok(Some(table))) => {
+            Message::PlotStyleLoaded(Ok(Some((table, note)))) => {
                 if table.is_stb {
                     self.command_line.push_error(
                         crate::t!(
@@ -8364,6 +8371,11 @@ impl OpenCADStudio {
                     )
                     .as_ref(),
                 );
+                // Non-fatal registration feedback — e.g. the picked file was
+                // already registered and the existing table was kept.
+                if let Some(note) = note {
+                    self.command_line.push_info(&note);
+                }
                 self.active_plot_style = Some(table);
                 self.plot_dialog.plot_styles = crate::io::plot_style::available_ctb_names();
                 Task::none()