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/io/pdf_export.rs

@@ -62,7 +62,7 @@ pub async fn pick_pdf_path_owned(stem: String) -> Option<std::path::PathBuf> {
         .add_filter("All Files", &["*"])
         .save_file()
         .await
-        .map(|h| h.path().to_path_buf())
+        .map(|h| crate::sys::handle_path(&h))
 }
 
 // ── PDF builder ───────────────────────────────────────────────────────────