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/main.rs

@@ -15,6 +15,7 @@ mod patterns;
 mod scene;
 mod snap;
 mod ui;
+mod par;
 mod sys;
 mod update_check;