style: use one spelling across the codebase

The repo mixed variants such as center/centre and color/colour, so a
name could be spelled either way and searches missed half the hits.
Standardize on the US spelling, matching .NET's own APIs (Color, Center).

Wire values owned by other systems keep their spelling: the material
ledger's "Cancelled" hold status and GitHub's "cancelled" job status.

No behavior change beyond the spelling of a few status messages.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Tests/CNC/ProgramSubProgramTransformTests.cs

@@ -12,7 +12,7 @@ public class ProgramSubProgramTransformTests
 {
     private const double QuarterTurn = System.Math.PI / 2;
 
-    /// <summary>Incremental hole sub-program: a lead-in from the hole centre to
+    /// <summary>Incremental hole sub-program: a lead-in from the hole center to
     /// (1, 0), then a full radius-1 circle.</summary>
     private static Program MakeHoleSub()
     {
@@ -58,7 +58,7 @@ public class ProgramSubProgramTransformTests
         pgm.Rotate(QuarterTurn);
 
         // (5, 5) -> (-5, 5) and (15, 5) -> (-5, 15); each lead-in ends one unit along
-        // the rotated +X axis, i.e. at +Y from its centre.
+        // the rotated +X axis, i.e. at +Y from its center.
         var ends = LeadInEnds(pgm);
         Assert.Equal(2, ends.Count);
         AssertPoint(-5, 6, ends[0]);