Display tessellation plans (3/4): display meshes from plans, GPU evaluation in the viewport (#805)

Display meshes are built from tessellation plans, with GPU evaluation in the viewport.

crates/doc/src/eval.rs

@@ -5,6 +5,7 @@ use std::hash::{Hash, Hasher};
 use std::sync::{Arc, OnceLock};
 
 use serde::Serialize;
+use solidcraft_geom::tess::TessPlan;
 use solidcraft_geom::{Mesh, Plane, Region2, Vec2, Vec3};
 use solidcraft_kernel::{self as kernel, Body, BoolOp};
 use solidcraft_sketch::{CurveKind, Profile, SolveReport, find_profiles};
@@ -27,6 +28,9 @@ pub struct ModelBody {
     // Timeline snapshots are cloned before meshes are requested. Share the cell itself so
     // meshing any snapshot also warms every unchanged copy of this body.
     mesh: Arc<OnceLock<Arc<Mesh>>>,
+    /// The display plan the mesh is evaluated from (none when the body has no plan: a mesh
+    /// body, or one the planner refuses), shared like the mesh. A GPU viewport draws it.
+    plan: Arc<OnceLock<Option<Arc<TessPlan>>>>,
     /// Identity of this body's content for the evaluation cache: the key of the evaluation
     /// that produced it (kept while later features pass it through unchanged).
     key: u64,
@@ -51,6 +55,7 @@ impl ModelBody {
             body,
             feature,
             mesh: Arc::new(OnceLock::new()),
+            plan: Arc::new(OnceLock::new()),
             key: 0,
             names: Default::default(),
             made: Default::default(),
@@ -68,12 +73,34 @@ impl ModelBody {
     pub fn mesh(&self) -> Arc<Mesh> {
         self.mesh
             .get_or_init(|| {
-                let tol = (self.body.size() * 1e-3).max(1e-3);
-                Arc::new(self.body.display_mesh_angular(tol, kernel::DISPLAY_ANGLE).unwrap_or_default())
+                let mesh = match self.display_plan() {
+                    Some(plan) => self.body.display_mesh_of(&plan),
+                    None => self.body.reference_display_mesh_angular(self.display_tol(), kernel::DISPLAY_ANGLE),
+                };
+                Arc::new(mesh.unwrap_or_default())
             })
             .clone()
     }
 
+    /// The display plan behind [`ModelBody::mesh`] (same tolerance and angle), made once; `None`
+    /// when the body has no plan and its mesh comes from the reference mesher.
+    pub fn display_plan(&self) -> Option<Arc<TessPlan>> {
+        self.plan
+            .get_or_init(|| match self.body.display_plan_angular(self.display_tol(), kernel::DISPLAY_ANGLE) {
+                Ok(plan) => Some(Arc::new(plan)),
+                Err(e) => {
+                    log::debug!("{}: no display plan ({e})", self.name);
+                    None
+                }
+            })
+            .clone()
+    }
+
+    /// The display mesh's chord tolerance: 1/1000 of the body's size.
+    fn display_tol(&self) -> f64 {
+        (self.body.size() * 1e-3).max(1e-3)
+    }
+
     /// Already computed display geometry; saving must not trigger cold tessellation.
     pub fn cached_mesh(&self) -> Option<Arc<Mesh>> {
         self.mesh.get().cloned()