perf: parallel wire tessellation with Rayon (Option C)

Extract tessellate_one body into a Send-compatible free function
tessellate_entity, then drive it with rayon::into_par_iter() in
wires_for_block(). Spreads file-open and post-edit tessellation across
all CPU cores. Navigation frames are unaffected (Options A/B cache
eliminates per-frame tessellation entirely).

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

Cargo.lock

@@ -14,6 +14,7 @@ dependencies = [
  "image",
  "open",
  "printpdf",
+ "rayon",
  "rfd",
  "truck-meshalgo",
  "truck-modeling",