eachValid

Runs fn(value) for every valid element of one chunk in index order, word-wise over the validity bitmap (ADR-2026-10-18-11): sentinel chunks (all valid) walk the raw values - the dense, auto-vectorisable path with no per-element null test - and bitmap chunks branch once per 64-bit window: an all-valid window takes the dense walk, a partially valid one walks only its set bits. Invalid lanes are skipped, never masked off by arithmetic (arbitrary NaN or infinite payloads multiplied by zero are not a null mask). The element order is the plain scan order, so accumulator association and first-wins extrema are unchanged bit for bit; float loops do not reassociate (no fast-math).