feat(wasm): sequential rayon shim for the web build

wasm32 has no threads, so rayon's global pool would panic at runtime.
Add `crate::par::prelude`: native re-exports `rayon::prelude`, wasm
provides sequential `std`-iterator equivalents (par_iter / into_par_iter
/ flat_map_iter over slices, Vec, HashMap, HashSet). All call sites now
`use crate::par::prelude::*;` instead of rayon directly.

- wire_gpu's rayon `reduce` (2-arg, no std equivalent) is cfg-split to
  a sequential flat_map concat on wasm.
- rayon moves to a non-wasm target dependency.

Native still runs in parallel; all 75 tests pass. wasm bin compiles.

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

src/io/mod.rs

@@ -602,7 +602,7 @@ pub(crate) fn is_entity_corrupt(e: &EntityType) -> bool {
 }
 
 pub fn purge_corrupt_entities(doc: &mut CadDocument) -> usize {
-    use rayon::prelude::*;
+    use crate::par::prelude::*;
     // Detection is pure and read-only; the per-vertex finite/extent checks on
     // large polylines dominate, so fan the scan out across cores. Gather
     // entity references in one pass, test in parallel, then remove serially