feat(posts): trigger the Cincinnati safety head raise by rapid length

The safety raise (M47 P<SafetyHeadraiseDistance>) used to fire only when
the next part had a different drawing name, which has nothing to do with
crash risk: a long rapid between copies of the same part got a plain M47,
while a short hop between neighbouring different drawings got the full
raise. Sheets are rarely flat (5/8" plate can be ~1.5" out of flat within
supplier tolerance), and longer rapids cross more of that bow.

Add SafetyHeadraiseRapidDistance (program units, default 12"): any rapid to
the next pierce longer than it takes the safety raise, regardless of
drawing. Unknown travel never does.

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

Posts/OpenNest.Posts.Cincinnati/CincinnatiPartSubprogramWriter.cs

@@ -89,7 +89,6 @@ public sealed class CincinnatiPartSubprogramWriter
                 PartName = drawingName,
                 IsFirstFeatureOfPart = false,
                 IsLastFeatureOnSheet = isLastFeature,
-                IsSafetyHeadraise = false,
                 IsExteriorFeature = exterior[i],
                 IsEtch = isEtch,
                 LibraryFile = isEtch ? etchLibrary : cutLibrary,