With DIMASSOC on, every chained dimension looked up the objects under
its definition points by scanning the whole space, and each point adds
an entity, so a long `points` list took quadratic time (16,000 points:
2.7 s continue, 10 s baseline). The chain now builds one x-sorted
SnapIndex up front. Dimensions are no snap targets, so the index stays
valid while the chain adds them, and each lookup is a binary search. The
index picks the same snap as find_snap: lowest rank first, then the first
object in space order. A call with more than 100,000 points is refused.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
