feat(draworder): cross-type draw order + context menu

DRAWORDER now affects rendering immediately (bump geometry epoch so the
sort cache rebuilds) and front/back anchor to the block's full effective
sort range instead of only existing table entries, so bring-to-front
actually lands on top. The render fallback sorts table-absent entities by
their own handle.

Draw order now applies across entity types, not just within a type. A
per-entity draw-order rank (SortEntitiesTable override or own handle,
ranked per block) is encoded as a small signed clip-z bias in the wire,
hatch, image, and 3DFACE pipelines. The shared LessEqual depth buffer then
orders fills, lines, rasters and 2D solids against each other regardless of
pass order — without touching model coordinates. 3D meshes keep real
geometric depth; wipeouts stay always-on-top.

Adds a Draw Order submenu to the viewport context menu (Bring to Front,
Send to Back, Bring Above/Send Under Object). The above/under items run an
interactive reference-object pick, then reorder the captured selection.

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

src/app/update.rs

@@ -3006,6 +3006,7 @@ impl OpenCADStudio {
                 if sel.right_down {
                     if !sel.right_dragging {
                         sel.context_menu = sel.last_move_pos;
+                        sel.draworder_submenu = false;
                     }
                     sel.right_down = false;
                     sel.right_press_pos = None;
@@ -3379,6 +3380,29 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::DrawOrderSubmenuToggle => {
+                let i = self.active_tab;
+                let mut sel = self.tabs[i].scene.selection.borrow_mut();
+                sel.draworder_submenu = !sel.draworder_submenu;
+                Task::none()
+            }
+
+            Message::DrawOrderPickRef(above) => {
+                let i = self.active_tab;
+                self.tabs[i].scene.selection.borrow_mut().context_menu = None;
+                let to_move: Vec<_> = self.tabs[i].scene.selected.iter().cloned().collect();
+                if to_move.is_empty() {
+                    self.command_line
+                        .push_error("DRAWORDER: select entities first.");
+                } else {
+                    use crate::command::CadCommand;
+                    let cmd = super::commands::DrawOrderRefCommand::new(to_move, above);
+                    self.command_line.push_info(&cmd.prompt());
+                    self.tabs[i].active_cmd = Some(Box::new(cmd));
+                }
+                Task::none()
+            }
+
             Message::SelectSimilar => {
                 let i = self.active_tab;
                 self.tabs[i].scene.selection.borrow_mut().context_menu = None;