Windowed Map

Sliding-window iterator. For a sorted source `[s_0, s_1, …, s_{n-1}]` of length n and `windowSize: k`, evaluates `windowBody` once for each `i ∈ [0, n-k]` with `windowVar` bound to the list `[s_i, s_{i+1}, …, s_{i+k-1}]`. Stride is always 1 — every consecutive window is visited. If n < k, no emissions (silent empty result; matches the over-empty-source behavior of every other iterator). The body accesses individual window positions via `tx.ListItemAt`. Use `tx.PairwiseMap` instead for the common windowSize-2 case — cleaner authoring with named first/second binders.