Pairwise Map

Ergonomic shortcut for the dominant `WindowedMap windowSize: 2` case. For a source `[s_0, s_1, …, s_{n-1}]`, evaluates `pairBody` once per consecutive pair with `firstVar` bound to s_i and `secondVar` bound to s_{i+1}. Emits n - 1 results in source order. Source of length 0 or 1 emits nothing (silent empty, matches WindowedMap). The canonical use case is rendering pairwise diffs between consecutive items in a sorted sequence — changelogs, audit- trail diffs, time-series deltas, schedule gap rendering, state-transition rendering. Body composes with the existing iterator family (Filter, Concat, etc.) for set-difference and other comparative computations.