detaching: bottom chain-up not robust!

When bottom is broken, it becomes unrealistic to try to chain them all up by checking radial overlap.
It's much thinner than top-edges (hos).

So compared to detecting multiple strings of hos, detecting multiple bottom threads in multi-pieces is more challenging.

One way to chain bottoms up is to check angular overlap. Sort all bottom, count possible gaps into the angular span AS, in other words, only look at the start and end angles, see if it approaches the limit of the bbox range. But also count accumulated angular span AAS. If AAS(R) >> bbox_span(R), it's an attached groove.

Prerequisite: Bottom is captured in its relative entirety. So that we have enough stuff to accumulate.

Then how to get the strings of different revolutions within the attached-CC?