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/scene/image_model.rs

@@ -27,6 +27,10 @@ pub struct ImageModel {
     /// viewport clipping. Mirrors `WireModel.vp_scissor` /
     /// `HatchModel.vp_scissor`.
     pub vp_scissor: Option<[f32; 4]>,
+    /// Normalized draw-order depth in (0,1); higher draws on top. Fed to the
+    /// image pipeline as a small clip-z bias so the raster orders correctly
+    /// against other entity types.
+    pub draw_depth: f32,
 }
 
 impl ImageModel {
@@ -61,6 +65,7 @@ impl ImageModel {
             opacity,
             corners,
             vp_scissor: None,
+            draw_depth: 0.0,
         })
     }
 }