mesh: a node move measures its region in parallel and can reuse its baseline

move_nodes_with takes the region's quality before the move from an earlier run
and a coarser guard bisection, for a live preview; each pass measures the
elements in parallel (fixed blocks). The relaxation's state sizes its elements
(a certified check each) only for the strong mode, which alone uses the size:
on the crank arm's hex20 mesh that was 114 ms of every 130 ms preview.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/mesh/Cargo.toml

@@ -10,6 +10,7 @@ workspace = true
 
 [dependencies]
 fieldcraft-model = { workspace = true }
+fieldcraft-parallel = { workspace = true }
 # Pure-Rust maths (a port of musl's libm) for sin, cos, log, exp, pow, hypot, acos: the same
 # bits on every platform, where the system library's are not correctly rounded and differ (the
 # plate's hex mesh differed between macOS and Linux, #438). atan2 and cbrt are crate::vec3's.