perf(render): batch solid meshes, fix per-pick freezes on heavy 3D models

Opening / navigating a heavy 3D DWG (e.g. a BIM model with ~10k ACIS solids)
was effectively frozen. Several independent costs, all addressed here:

- Face3D edge/fill buffers were re-uploaded on every camera move (keyed on
  camera_generation). They are world-space and selection-independent, so gate
  them on (geometry_epoch, fill_mode) instead — a pan no longer re-walks every
  wire.
- Solid meshes were drawn one buffer + one draw call per solid (~10k draws/
  frame), which strangled the GPU front end. Concatenate every solid's LOD0
  into a few large buffers (split only to stay under the 256 MB buffer cap) and
  draw the whole set in a handful of calls. Built once per geometry epoch, so
  selection / hover never re-pack it.
- Selection / hover highlight is now a small tinted overlay of just the picked
  solids, drawn last with an Always-depth pipeline so it shows on top even when
  the solid is occluded — instead of re-tinting and re-uploading all meshes.
- Hover / pick hit-testing scanned every triangle of every solid each move
  (seconds on a heavy model). Add a 3D-AABB screen-rect broad phase
  (MeshLodSet now carries z_aabb) and reuse the renderer's cached expanded mesh
  set, so picking is O(solids) cheap projections plus the few solids actually
  under the cursor.
- Block-internal hatch hit-testing exploded every INSERT on every hover.
  Cache the result per geometry epoch and skip blocks that contain no hatch.
- Curved-surface (cone/sphere/torus) tessellation tolerance is now radius-
  relative (CURVE_REL_TOL) so a cylinder's facet count is size-independent and
  matches the circle/arc wire tessellation, instead of exploding on large radii.

Also adds docs/tessellation.md mapping every EntityType to its tessellation
path (truck B-rep vs truck curve topology vs direct).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/convert/solid3d_tess.rs

@@ -181,8 +181,7 @@ fn tessellate_sat_lods(
     if lods.is_empty() {
         return None;
     }
-    let world_aabb = mesh_aabb(&lods[0]);
-    Some(MeshLodSet { lods, world_aabb })
+    Some(MeshLodSet::from_lods(lods))
 }
 
 /// Extract the body's placement transform from the SAT document: a row-major
@@ -243,6 +242,7 @@ fn scale_lod(base: LodConfig, facet_res: f64) -> LodConfig {
 
 /// World-XY AABB of the mesh — used by the render-pipeline LOD selector
 /// to pick a level based on projected pixel diagonal.
+#[allow(dead_code)] // superseded by mesh_model::compute_mesh_aabb (3D); kept for reference
 pub(crate) fn mesh_aabb(mesh: &MeshModel) -> [f32; 4] {
     let mut min_x = f32::INFINITY;
     let mut min_y = f32::INFINITY;