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 <[email protected]>

src/sys.rs

@@ -0,0 +1,31 @@
+// Small platform shims for things the desktop build does natively but the web
+// (wasm) build must handle differently or skip.
+
+/// Open a URL in the user's browser. On the desktop this launches the default
+/// handler; on the web the app already *is* in a browser, so for now this is a
+/// no-op (a real implementation would call `window.open`).
+#[cfg(not(target_arch = "wasm32"))]
+pub fn open_url(url: &str) {
+    let _ = open::that(url);
+}
+
+#[cfg(target_arch = "wasm32")]
+pub fn open_url(_url: &str) {
+    // TODO(web): web_sys::window().open_with_url(_url)
+}
+
+/// Turn an `rfd` file handle into a `PathBuf` the rest of the app keys on.
+///
+/// Desktop returns the real filesystem path. The browser has no path, so we
+/// synthesize one from the file name — enough for the app to compile and track
+/// the document name; actual byte I/O on the web reads the handle directly
+/// (a follow-up).
+#[cfg(not(target_arch = "wasm32"))]
+pub fn handle_path(h: &rfd::FileHandle) -> std::path::PathBuf {
+    h.path().to_path_buf()
+}
+
+#[cfg(target_arch = "wasm32")]
+pub fn handle_path(h: &rfd::FileHandle) -> std::path::PathBuf {
+    std::path::PathBuf::from(h.file_name())
+}