Column.interpolate

Interpolates null gaps linearly over the implicit row positions (ADR-2026-10-30-01): a null slot strictly between two valid observations at positions i0 and i1 takes lo + (hi - lo) * (i - i0) / (i1 - i0), computed in double precision and rounded once to the output dtype. Leading and trailing null runs stay null (no bounding observation on one side); a column with no nulls returns unchanged in its own dtype; a column with at most one valid observation stays as it is (under the promoted dtype for integers). NaN is data: a NaN endpoint propagates into the gap. Chunk boundaries never bound a gap (the walk runs over a rechunked view), and the kernel is serial so the arithmetic association is fixed.