ship: apply scale + gate approximate external parts behind a toggle

Two changes after user feedback that externals sit near-but-wrong (shield inset
into the hull, thruster floating close):

- Scale: the joint table's slots 5–7 are per-axis scale (a GN_ShieldG frame ships
  at 0.5, GN_Jet FX at 2.4–6.5) and were being ignored, rendering scaled parts at
  the wrong size. ScenePart now carries `s` and applies R·(S·v)+T.
- External placement is fundamentally approximate: a GN_* frame is the mount
  PIVOT (f105's two GN_ShieldG frames are both on the centreline), while the
  part's outboard/rotational offset lives in a detail sub-rig / runtime code this
  static pass can't recover. So assemble_ship gains `include_external`: the hull
  tier (rou_ nodes) is exact and always drawn; the external tier (bridge / shield
  / engine at GN_ frames) is opt-in via a "Show external parts" checkbox in the
  Ships browser, off by default so the clean, correct hull is the default view.

The module doc is corrected to the scene-graph mechanism (the old "draw parts
untransformed" claim was wrong).

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

crates/sylpheed-viewer/src/ui.rs

@@ -1589,9 +1589,18 @@ fn draw_ships_ui(
                     ships.filter.clear();
                 }
             });
+            ui.horizontal(|ui| {
+                ui.checkbox(&mut ships.show_external, "Show external parts");
+                ui.label(
+                    egui::RichText::new("(bridge / shield gens / engines — approximate placement)")
+                        .weak()
+                        .small(),
+                );
+            });
             ui.separator();
 
-            let ShipBrowser { rows, filter, faction, selected, .. } = &mut *ships;
+            let ShipBrowser { rows, filter, faction, selected, show_external, .. } = &mut *ships;
+            let show_external = *show_external;
             let needle = filter.to_lowercase();
             let mut to_render: Option<(String, String, String)> = None;
 
@@ -1677,7 +1686,7 @@ fn draw_ships_ui(
             if let Some((id, file, label)) = to_render {
                 let id2 = id.clone();
                 *selected = Some(id);
-                render.send(RequestShipRender { file, id: id2, label });
+                render.send(RequestShipRender { file, id: id2, external: show_external, label });
             }
         });
     ships.open &= open;