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/shaders/image.wgsl

@@ -19,13 +19,16 @@ struct Uniforms {
 
 // Per-image params (fade, clip flag, etc.)
 struct ImageParams {
-    opacity: f32,
-    _pad0: f32,
-    _pad1: f32,
-    _pad2: f32,
+    opacity:    f32,
+    draw_depth: f32,   // signed (-1,1) draw-order bias; 0 = neutral
+    _pad1:      f32,
+    _pad2:      f32,
 };
 @group(1) @binding(2) var<uniform> img_params: ImageParams;
 
+// Draw-order depth bias (see wire.wgsl).
+const DRAW_ORDER_BIAS: f32 = 0.001;
+
 // ── Vertex stage ──────────────────────────────────────────────────────────────
 struct VertIn {
     @location(0) pos:  vec3<f32>,
@@ -41,6 +44,7 @@ struct VertOut {
 fn vs_main(in: VertIn) -> VertOut {
     var out: VertOut;
     out.clip_pos = u.mvp * vec4<f32>(in.pos, 1.0);
+    out.clip_pos.z = out.clip_pos.z - img_params.draw_depth * DRAW_ORDER_BIAS * out.clip_pos.w;
     out.uv = in.uv;
     return out;
 }