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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

Cargo.lock

@@ -1433,6 +1433,7 @@ name = "fieldcraft-mesh"
 version = "0.6.0"
 dependencies = [
  "fieldcraft-model",
+ "fieldcraft-parallel",
  "libm",
  "num-bigint",
  "num-rational",