Support curvilinear and projected grids in GridToSparseInterpolation.

[copybara-service]

Support curvilinear and projected grids in GridToSparseInterpolation.

• Generalize GridToSparseInterpolation to support 2D curvilinear/projected grids.
• Precompute Delaunay simplex indices and barycentric weights, caching geometry for fast repeated evaluation.
• Add unit tests for 2D curvilinear interpolation correctness and out-of-bounds masking.

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[sylvesterkaczmarek]

The interpolation cache key is not geometry-safe: it includes only array shapes plus the first grid/target latitude, and omits longitudes and the remaining coordinates. Reusing one interpolator with a same-sized reference whose station coordinates changed can silently reuse stale corners and weights. Could the key include the full grid/target geometry (or a stable fingerprint/identity)? A two-call test with the same shapes and first latitude but a changed second point or longitude would expose this.