feat(wasm): compile the library for wasm32 (issue #45 groundwork)

First step toward a web build: the library now compiles for
wasm32-unknown-unknown with --no-default-features.

- Add a `solid3d` feature (default on) gating truck-meshalgo /
  truck-shapeops / lzma-sys. These pull vtkio → xz2 → lzma-sys (C),
  which cannot cross-compile to wasm. Without the feature the 3-D
  solid path (tessellation, booleans, ACIS import) becomes no-ops;
  2-D CAD is unaffected. Native builds keep the feature by default.
- Move native-only crates (ureq, open) to non-wasm target deps and
  add cfg shims: update-check is a no-op on wasm, and a new `sys`
  module provides open_url + handle_path (rfd FileHandle → PathBuf;
  synthesizes a name-based path on the web).

Native build + all 75 tests unchanged. Browser runtime wiring
(entry point, trunk, real file I/O) is a follow-up.

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

src/scene/convert/mod.rs

@@ -1,5 +1,21 @@
 pub mod acad_to_truck;
+#[cfg(feature = "solid3d")]
 pub mod acis_to_truck;
+/// Without `solid3d` (e.g. wasm) ACIS/SAT meshing is unavailable; the entry
+/// point stays so callers compile, returning no mesh.
+#[cfg(not(feature = "solid3d"))]
+pub mod acis_to_truck {
+    use crate::scene::model::mesh_model::MeshLodSet;
+    use acadrust::entities::acis::SatDocument;
+    pub fn tessellate_sat_truck(
+        _sat: &SatDocument,
+        _name: String,
+        _color: [f32; 4],
+        _facet_res: f64,
+    ) -> Option<MeshLodSet> {
+        None
+    }
+}
 pub mod truck_tess;
 pub mod tessellate;
 pub mod tess_util;

src/scene/convert/truck_tess.rs

@@ -8,9 +8,11 @@
 // The caller (acad_to_truck / tessellate) decides which function to call based on
 // the truck topology level of the object.
 
+#[cfg(feature = "solid3d")]
 use truck_meshalgo::tessellation::{MeshableShape, MeshedShape};
 use truck_modeling::base::{BoundedCurve, ParameterDivision1D};
 use truck_modeling::{Edge, Shell, Solid, Vertex, Wire};
+#[cfg(feature = "solid3d")]
 use truck_polymesh::PolygonMesh;
 
 
@@ -140,21 +142,45 @@ pub fn tessellate_wire(w: &Wire, offset: [f64; 3]) -> TruckTessResult {
 // ── Shell ─────────────────────────────────────────────────────────────────
 
 #[allow(dead_code)]
+#[cfg(feature = "solid3d")]
 pub fn tessellate_shell(s: &Shell, offset: [f64; 3]) -> TruckTessResult {
     let meshed = s.triangulation(MESH_TOL);
     polygon_to_result(meshed.to_polygon(), offset)
 }
 
+/// Without `solid3d` (e.g. wasm) there is no mesher; a shell yields no mesh.
+#[cfg(not(feature = "solid3d"))]
+pub fn tessellate_shell(_s: &Shell, _offset: [f64; 3]) -> TruckTessResult {
+    TruckTessResult::Mesh {
+        verts: Vec::new(),
+        normals: Vec::new(),
+        indices: Vec::new(),
+    }
+}
+
 // ── Solid ─────────────────────────────────────────────────────────────────
 
 #[allow(dead_code)]
+#[cfg(feature = "solid3d")]
 pub fn tessellate_solid(s: &Solid, offset: [f64; 3]) -> TruckTessResult {
     let meshed = s.triangulation(MESH_TOL);
     polygon_to_result(meshed.to_polygon(), offset)
 }
 
+/// Without `solid3d` (e.g. wasm) there is no mesher; a solid yields no mesh.
+#[allow(dead_code)]
+#[cfg(not(feature = "solid3d"))]
+pub fn tessellate_solid(_s: &Solid, _offset: [f64; 3]) -> TruckTessResult {
+    TruckTessResult::Mesh {
+        verts: Vec::new(),
+        normals: Vec::new(),
+        indices: Vec::new(),
+    }
+}
+
 // ── Internal ─────────────────────────────────────────────────────────────
 
+#[cfg(feature = "solid3d")]
 fn polygon_to_result(mesh: PolygonMesh, offset: [f64; 3]) -> TruckTessResult {
     let verts: Vec<[f32; 3]> = mesh
         .positions()

src/scene/model/solid_model.rs

@@ -130,10 +130,12 @@ pub enum Bool {
     Intersect,
 }
 
+#[cfg(feature = "solid3d")]
 const BOOL_TOL: f64 = 0.05;
 
 /// Combine two solids. `Subtract` removes `b` from `a`. Returns `None` when the
 /// operation fails (e.g. the solids don't actually overlap).
+#[cfg(feature = "solid3d")]
 pub fn boolean(op: Bool, a: &Solid, b: &Solid) -> Option<Solid> {
     match op {
         Bool::Union => truck_shapeops::or(a, b, BOOL_TOL),
@@ -146,6 +148,12 @@ pub fn boolean(op: Bool, a: &Solid, b: &Solid) -> Option<Solid> {
     }
 }
 
+/// Without `solid3d` (e.g. wasm) there is no boolean kernel.
+#[cfg(not(feature = "solid3d"))]
+pub fn boolean(_op: Bool, _a: &Solid, _b: &Solid) -> Option<Solid> {
+    None
+}
+
 // ── Tessellation ────────────────────────────────────────────────────────────
 
 /// Tessellate a truck `Solid` into a single-LOD `MeshLodSet` (world-space,