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/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,