The far edge of a stepped slab (the operand cap edge where a segment's
wall ends) was recorded by testing whether each slab segment runs along
an ending cap's segment in 2D, a tolerance comparison of coordinates.
Matching by topology makes the far edge exact by construction instead:
when the wall's top is the ending level, nothing crosses it there, and it
meets the caps at that level in exactly one edge, that edge is over every
segment beside the wall, with no position compared.
A vertex walk up the wall's side edges from the segment's ends was tried
first and does not settle the cases that need it: a notched or stepped
wall continues past a segment's end (no side edge rises from it), and
walls split by earlier stations have mid-chain vertices with no side edge.
Whenever the walk succeeds, the wall is a rectangle over the segment and
the one-edge rule above already holds.
The positional match remains only where topology cannot say which edge
is over a segment: a wall that meets the ending caps in several edges (a
top split at earlier station vertices, or beside a wall crossing it at
that level) or that carries on past the level elsewhere. It is documented
in `Section::mark_far` and its pair budget no longer stops topological
marking. Far edges are also only marked on the slab that actually ends at
the level (a gap level no longer revisits an earlier slab).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J53sR6gEaVt56soFDpTck8
