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 <noreply@anthropic.com>

Cargo.toml

@@ -17,6 +17,7 @@ open = "5"
 printpdf = "0.9.1"
 flate2 = "1"
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "bmp", "tiff"] }
+rayon = "1"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 windows-sys = { version = "0.59", features = ["Win32_UI_Shell", "Win32_UI_WindowsAndMessaging"] }