Every lead was checked against every placed part's material and every
rapid against every completed contour, so each check cost O(parts) and
planning a plate cost O(parts^2). Lead checks were 88% of planning time
on a 144-part grid.
LeadMaterialSnapshot and each completed contour now keep a conservative
extent (an arc counts as its whole supporting circle). A lead or rapid
skips material or a contour only when the extents are farther apart
than 1e-6 x (1 + coordinate size), far above the contact tolerance, so
results are unchanged: anything touching or closer still gets the full
native check. The rapid filter also applies to pre-post verification,
which shares ReleasedContourState.
New tests cover the cases just inside the skip: an arc lead and a
completed arc whose bulge reaches past their endpoints, and a lead and
a rapid that only touch another part's extent.
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