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/pipeline/image_gpu.rs

@@ -45,7 +45,10 @@ impl ImageVertex {
 #[derive(Copy, Clone, bytemuck::Pod, bytemuck::Zeroable)]
 struct ImageParams {
     opacity: f32,
-    _pad: [f32; 3],
+    /// Signed draw-order depth (-1,1); applied as a clip-z bias in the shader
+    /// so the raster orders against other entity types. 0.0 = neutral.
+    draw_depth: f32,
+    _pad: [f32; 2],
 } // 16 bytes
 
 // ── Per-image GPU handle ──────────────────────────────────────────────────
@@ -118,7 +121,8 @@ impl ImageGpu {
         // ── Opacity uniform ───────────────────────────────────────────────
         let params = ImageParams {
             opacity: model.opacity.clamp(0.0, 1.0),
-            _pad: [0.0; 3],
+            draw_depth: model.draw_depth,
+            _pad: [0.0; 2],
         };
         let _params_buf = device.create_buffer_init(&wgpu::util::BufferInitDescriptor {
             label: Some("image.params"),