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/update_check.rs

@@ -28,6 +28,7 @@ pub struct UpdateInfo {
     pub body: String,
 }
 
+#[cfg(not(target_arch = "wasm32"))]
 pub async fn check_for_update() -> Option<UpdateInfo> {
     std::thread::spawn(fetch_latest_if_outdated)
         .join()
@@ -35,6 +36,14 @@ pub async fn check_for_update() -> Option<UpdateInfo> {
         .flatten()
 }
 
+/// On wasm there is no background thread or blocking HTTP client; the web build
+/// skips the self-update check (the page is always served fresh).
+#[cfg(target_arch = "wasm32")]
+pub async fn check_for_update() -> Option<UpdateInfo> {
+    None
+}
+
+#[cfg(not(target_arch = "wasm32"))]
 fn fetch_latest_if_outdated() -> Option<UpdateInfo> {
     let agent: ureq::Agent = ureq::Agent::config_builder()
         .timeout_global(Some(std::time::Duration::from_secs(5)))