feat(render): HiddenLine mode via depth-only prepass

3D yüzeyleri (Solid3D mesh, 3DFACE quad, PolyfaceMesh/PolygonMesh face
tris) HiddenLine modunda renk yazmadan z-buffer'a yazıyor; ardından
kenarlar wireframe pipeline ile çizilince LessEqual depth testi
arkadaki segmentleri eliyor. 2D fill'ler (text greek, MultiLeader
arka planı) ayrı bir vertex buffer'a ayrıldı ve her modda renkli
çizilmeye devam ediyor.

- Face3DGpu: vertex_buffer artık 3D / 2D olarak ikiye ayrıldı
- mesh_depth_pipeline + face3d_depth_pipeline (write_mask=empty)
- RenderModeFlags.hidden_line + Primitive.hidden_line plumbing

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/scene/render.rs

@@ -114,6 +114,10 @@ pub struct Primitive {
     /// `*WithEdges` variants and the pure wireframes leave it on.
     #[allow(dead_code)]
     pub(super) show_3d_edges: bool,
+    /// HiddenLine routes 3D fills through a depth-only prepass so edges
+    /// occluded by closer geometry are culled by the LessEqual depth
+    /// test on the wire passes that follow.
+    pub(super) hidden_line: bool,
     pub(super) geometry_epoch: u64,
     /// Camera generation captured when this Primitive was assembled. Paired
     /// with `geometry_epoch` so the wire buffers re-upload when the view
@@ -124,15 +128,16 @@ pub struct Primitive {
 /// Flags the render pipeline consumes, derived from
 /// [`acadrust::entities::ViewportRenderMode`]. Each shaded variant fills
 /// 3D faces and meshes; the pure wireframes drop the fill and keep only
-/// edges. `*WithEdges` variants render both. `HiddenLine` is currently
-/// rendered as plain solid (until Phase 2 wires up the depth-only
-/// prepass); `FlatShaded` vs `GouraudShaded` differ in shader uniform
-/// only and produce identical fill flags here.
+/// edges. `*WithEdges` variants render both. HiddenLine uses a depth
+/// prepass: face/mesh fills are uploaded but routed through depth-only
+/// pipelines so hidden edges drop out. `FlatShaded` vs `GouraudShaded`
+/// differ in shader uniform only and produce identical fill flags here.
 #[derive(Clone, Copy, Debug)]
 pub struct RenderModeFlags {
     pub face3d_fill: bool,
     pub mesh_fill: bool,
     pub show_3d_edges: bool,
+    pub hidden_line: bool,
 }
 
 pub fn render_mode_flags(
@@ -144,21 +149,25 @@ pub fn render_mode_flags(
             face3d_fill: false,
             mesh_fill: false,
             show_3d_edges: true,
+            hidden_line: false,
         },
         M::HiddenLine => RenderModeFlags {
             face3d_fill: true,
             mesh_fill: true,
             show_3d_edges: true,
+            hidden_line: true,
         },
         M::FlatShaded | M::GouraudShaded => RenderModeFlags {
             face3d_fill: true,
             mesh_fill: true,
             show_3d_edges: false,
+            hidden_line: false,
         },
         M::FlatShadedWithEdges | M::GouraudShadedWithEdges => RenderModeFlags {
             face3d_fill: true,
             mesh_fill: true,
             show_3d_edges: true,
+            hidden_line: false,
         },
     }
 }
@@ -251,7 +260,14 @@ impl shader::Primitive for Primitive {
         // the draw path so meshes use the wireframe pipeline + the
         // pre-built triangle-edge index buffer.
         let mesh_wireframe = !self.mesh_fill;
-        pipeline.render(encoder, target, *clip, self.bg_color, mesh_wireframe);
+        pipeline.render(
+            encoder,
+            target,
+            *clip,
+            self.bg_color,
+            mesh_wireframe,
+            self.hidden_line,
+        );
         if self.show_viewcube {
             pipeline.viewcube.render(encoder, target, *clip);
         }
@@ -455,6 +471,7 @@ impl Scene {
             view_wireframe,
             mesh_fill: flags.mesh_fill,
             show_3d_edges: flags.show_3d_edges,
+            hidden_line: flags.hidden_line,
             geometry_epoch: self.geometry_epoch,
             camera_generation: self.camera_generation,
         }
@@ -505,6 +522,7 @@ impl Scene {
             view_wireframe,
             mesh_fill: flags.mesh_fill,
             show_3d_edges: flags.show_3d_edges,
+            hidden_line: flags.hidden_line,
             geometry_epoch: self.geometry_epoch,
             camera_generation: self.camera_generation,
         }