uber / h3-go / 27735272019

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perf: fold toVec3 inverse projection into precomputed trig + one sqrt

toVec3 (the inverse gnomonic projection behind CellToLatLng, CellToBoundary,
CellArea*, EdgeLength*, vertex/edge boundaries, and the polygon paths) computed
atan2 + atan + two sin/cos pairs per call. Replace them with algebra:
  - cos/sin of the in-plane angle taken directly from the vector (no atan2);
  - per-face azimuth and the Class III rotation folded in via angle-addition
    against precomputed cos/sin tables;
  - the inverse gnomonic via cos(atan r)=1/sqrt(1+r^2), sin(atan r)=r/sqrt(1+r^2)
    (one sqrt, no atan/sin/cos), dropping posAngleRads since only sin/cos are used.

Five transcendental calls become one sqrt. The transform is algebraically
exact; output differs from C only at the ULP level, well within the parity
suite's 1e-5 relative tolerance. toLatLng is left as-is (its asin/atan2 are the
returned angles). All white-box and parity tests pass.

benchstat, pure-Go before -> after (Apple M3 Max, count=10, benchtime=100ms):

                              │    sec/op    │   sec/op     vs base                │
CellToLatLng-16                  160.4n ± 2%   126.4n ± 1%  -21.20% (p=0.000 n=10)
CellLatLng-16                    160.6n ± 0%   126.4n ± 2%  -21.35% (p=0.000 n=10)
CellToBoundary-16                648.1n ± 1%   438.9n ± 2%  -32.28% (p=0.000 n=10)
CellBoundary-16                  647.2n ± 1%   463.8n ± 3%  -28.35% (p=0.000 n=10)
CellAreaRads2-16                 813.8n ± 0%   631.3n ± 3%  -22.43% (p=0.000 n=10)
CellAreaKm2-16                   814.5n ± 1%   612.9n ± 2%  -24.76% (p=0.000 n=10)
CellAreaM2-16                    823.4n ± 3%   610.4n ± 1%  -25.87% (p=0.000 n=10)
EdgeLengthRads-16                394.0n ± 1%   329.4n ± 3%  -16.38% (p=0.000 n=10)
EdgeLengthKm-16                  395.2n ± 3%   327.1n ± 3%  -17.22% (p=0.000 n=10)
EdgeLengthM-16                   397.2n ± 1%   331.0n ± 1%  -16.66% (p=0.000 n=10)
HexagonEdgeLengthAvgKm-16        7.874n ± 2%   ... (continued)

26 of 26 new or added lines in 1 file covered. (100.0%)

4247 of 4257 relevant lines covered (99.77%)

3180241.19 hits per line