One output fragment: a sub-triangle of an arranged input triangle, or an
untouched whole triangle. v is wound to match the input mesh’s outward
orientation; vi are the interned ids of the same three vertices.
Exact-point interner: one id per distinct point, with two disjoint key
spaces. f64-representable points (all input vertices, and any constructed
point that rounds exactly) key on their coordinate bits — no rational
hashing, so untouched input triangles intern for the cost of a HashMap
probe. Only genuinely non-representable constructed points use the
rational map. verts_f64 caches the correctly rounded approximation of
every id (exact for bit-keyed points), which downstream float filters
and output assembly reuse instead of re-rounding.
Canonical (sorted) edge between two interned vertex ids. Downstream
stages (classify rings, propagate flood fill) key their maps on these
integers instead of exact rational point pairs — vertex interning at
piece-emission time makes id equality coincide with exact geometric
identity.