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/mod.rs

@@ -308,6 +308,10 @@ pub enum Message {
     SaveDialogCancel,
     ClearScene,
     SetWireframe(bool),
+    /// Set the active tab's render mode (one of acadrust's seven visual
+    /// styles). Replaces the binary `SetWireframe` over time; the older
+    /// message stays for ribbon/CLI back-compat and forwards.
+    SetRenderMode(acadrust::entities::ViewportRenderMode),
     /// Switch camera projection: true = Orthographic, false = Perspective.
     SetProjection(bool),
     /// Select a ribbon module tab by index.