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h3: keep cell areas exact across the antimeridian (#176)

cagnoli subtracted the raw longitudes of an edge's endpoints. For an edge
crossing +-180 degrees the difference is close to +-360 and the true, tiny
longitude step is lost to the rounding of a value that large, so cells
straddling the antimeridian came out too small: 1e-8 relative at res 7,
0.1% at res 13 and 9% at res 15. Shifting both longitudes to the same side
of the antimeridian before subtracting keeps the subtraction between two
nearby values; sin and cos are 2pi-periodic, so nothing else changes. This
is upstream uber/h3#1274, which the pinned release does not yet contain.

The conformance suite could not see the defect: its seeded sample has
straddling cells only at resolutions 0 and 1, where the loss is far below
tolerance. The generator now carries the upstream change as a second patch
and writes floats/curated.jsonl, the upstream test pairs at res 13 to 15
plus the cell containing (lat, 180) at every resolution for three
latitudes, which the floats/cells checks run. The fifteen straddling res 0
and 1 cells already in floats/cells.jsonl move by at most 4e-14 relative;
every other file, the digests included, is byte for byte unchanged.

Three tests port the upstream ones: straddling cells agree with their
non-straddling neighbours, a cell on the antimeridian keeps its area when
rotated onto the prime meridian, and a small triangle across +-180 has the
area of the same triangle across 0. All three fail without the shift.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

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