After linesweeper 0.5 (#1028 on main), uniting the faces of a grid of
lines could split the merged strip again even with their corners shared
exactly: a sweep over seven or more faces moved shared vertices by about
1e-6 (within its eps) and the edges between faces no longer cancelled.
Faces cut by open paths now merge like closed shapes' faces already do
(FaceMerger::exact): in the planar map itself. build() is split into
map() (the graph and its faces) and build(); Map::merge traces the
boundary of the picked faces along the map's own edges, dropping the
edges between two picked faces and the loose ends inside them, so no
sweep runs on the result. FaceMerger finds each face again by a point
inside it, checks the area, and keeps the boolean union as a fallback.
Tests: the regression test merges through FaceMerger; a grid test
covers a ring with its hole, cells touching at a corner, a loose end,
and that the map path (not the fallback) produced the result.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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