feat(render): phase 0 — render-mode pick_list + 3-flag pipeline plumbing

Replaces the binary Wireframe/Solid toggle with a 7-mode dropdown
that maps onto acadrust's `ViewportRenderMode` enum: Wireframe 2D /
Wireframe 3D / Hidden Line / Flat Shaded / Gouraud Shaded / Flat
Shaded + Edges / Gouraud Shaded + Edges. Per-tab state lives on
`DocumentTab::render_mode`; the old `SetWireframe` message forwards
to `SetRenderMode` so the ribbon + WIREFRAME / SOLID command line
keep working.

`scene::render::render_mode_flags()` translates the mode into three
pipeline-facing booleans (`face3d_fill`, `mesh_fill`,
`show_3d_edges`); `mesh_fill` / `show_3d_edges` are stubs that the
upcoming phases will consume. `Primitive` carries the derived
`view_wireframe` plus the two new fields through to `prepare()`.

`Pipeline::upload_face3d` takes a `wireframe_only` flag that drops
3DFACE quad fills (built from `key_vertices`) while keeping the
`all_wires.fill_tris` channel — that channel carries text-LOD greek
rectangles and PolyfaceMesh face triangles which must stay visible
in wireframe mode. 3DFACE edges stay on regardless of mode.

Solid3D mesh wireframe rendering, Hidden Line z-prepass, and Flat /
Gouraud shader uniforms come in phases 1–4.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/document.rs

@@ -27,6 +27,7 @@ pub(super) struct DocumentTab {
     pub(super) selected_grips: Vec<GripDef>,
     pub(super) selected_handle: Option<Handle>,
     pub(super) wireframe: bool,
+    pub(super) render_mode: acadrust::entities::ViewportRenderMode,
     pub(super) visual_style: String,
     pub(super) last_cursor_world: glam::Vec3,
     pub(super) last_cursor_screen: iced::Point,
@@ -80,7 +81,8 @@ impl DocumentTab {
             selected_grips: vec![],
             selected_handle: None,
             wireframe: false,
-            visual_style: "Shaded".into(),
+            render_mode: acadrust::entities::ViewportRenderMode::Wireframe2D,
+            visual_style: "Wireframe 2D".into(),
             last_cursor_world: glam::Vec3::ZERO,
             last_cursor_screen: iced::Point::ORIGIN,
             history: HistoryState::default(),