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/scene/pipeline/wire_gpu.rs

@@ -24,7 +24,7 @@
 
 use crate::scene::model::wire_model::WireModel;
 use iced::wgpu;
-use rayon::prelude::*;
+use crate::par::prelude::*;
 
 /// Allocate a VERTEX buffer with `mapped_at_creation` and write `data` directly
 /// into the mapped slice. Skips the intermediate staging copy that
@@ -295,7 +295,7 @@ impl WireGpu {
         scissor: Option<[f32; 4]>,
         mesh_edge: bool,
     ) -> Vec<Self> {
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         const MAX_INSTANCES: usize = 268_435_456 / std::mem::size_of::<WireInstance>();
         let per: Vec<Vec<WireInstance>> = wires
             .par_iter()
@@ -340,6 +340,7 @@ impl WireGpu {
         // within a batch does not affect correctness. `reduce` concatenates the
         // per-wire Vec<WireInstance> chunks while letting rayon spread work
         // across cores.
+        #[cfg(not(target_arch = "wasm32"))]
         let instances: Vec<WireInstance> = wires
             .par_iter()
             .map(|wire| emit_wire_instances(wire, wire.color, wire_draw_depth(wire, depth_map)))
@@ -353,6 +354,12 @@ impl WireGpu {
                     acc
                 }
             });
+        // Web: no thread pool — concatenate the per-wire streams sequentially.
+        #[cfg(target_arch = "wasm32")]
+        let instances: Vec<WireInstance> = wires
+            .iter()
+            .flat_map(|wire| emit_wire_instances(wire, wire.color, wire_draw_depth(wire, depth_map)))
+            .collect();
 
         if instances.is_empty() {
             return vec![];