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/cache/block_cache.rs

@@ -125,7 +125,7 @@ impl BlockCache {
     /// because their entities are emitted as top-level wires, not via the
     /// cache.
     pub fn build(doc: &CadDocument, anno_scale: f32, bg_color: [f32; 4]) -> Self {
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         let mut cache = Self::new();
         let referenced = collect_referenced_blocks(doc);
         // Each defn is built independently: nested INSERTs are stored as
@@ -151,7 +151,7 @@ impl BlockCache {
     /// called in. Cycle guard: a self-referential block keeps an empty AABB
     /// (will fail every frustum test → not emitted, which is correct).
     fn compute_block_aabbs(&mut self, names: &[String]) {
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         // Phase 1 (parallel, read-only): each defn's union AABB is resolved
         // by `defn_aabb_recursive`, which only *reads* `self.defns` (the map
         // is fully built by now). There's no memoization, so a defn shared by

src/scene/mod.rs

@@ -155,7 +155,7 @@ pub fn build_derived_caches(doc: &CadDocument) -> DerivedCaches {
     // filters that `cache::block_cache::build_defn` already uses for block defns)
     // and prefer the entity-scan when the header center drifts more than
     // 10× its own half-span away from the entity centroid.
-    use rayon::prelude::*;
+    use crate::par::prelude::*;
 
     // Single pass over entities does triple duty: classify cache-kind handle
     // lists (hatch / image / mesh) AND accumulate per-entity centroids for the
@@ -2245,7 +2245,7 @@ impl Scene {
         };
 
         // Tessellate in parallel across all available CPU cores.
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         let doc = &self.document;
         // Selection / hover highlight is NOT baked into tessellation. It is
         // applied per frame in the GPU xray overlay pass from the live
@@ -4297,7 +4297,7 @@ impl Scene {
             })
             .collect();
 
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         self.hatches = entries
             .into_par_iter()
             .filter_map(|(handle, kind)| {
@@ -4350,7 +4350,7 @@ impl Scene {
             })
             .collect();
 
-        use rayon::prelude::*;
+        use crate::par::prelude::*;
         let facet_res = self.document.header.facet_resolution;
         let woff = self.world_offset;
         self.meshes = entries

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![];