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/mod.rs

@@ -702,6 +702,12 @@ pub enum Message {
     /// Extend the current selection with every entity in the active
     /// layout that matches a selected entity by (type, layer).
     SelectSimilar,
+    // ── Draw Order context menu ─────────────────────────────────────────
+    /// Toggle the Draw Order sub-items in the viewport context menu.
+    DrawOrderSubmenuToggle,
+    /// Begin an interactive reference-object pick to move the current
+    /// selection above (`true`) or below (`false`) the picked object.
+    DrawOrderPickRef(bool),
     /// Open the Quick Select panel. Initialises filters from the current
     /// selection's first entity (type + layer) when one is selected.
     QSelectOpen,