Function from_set_row 

pub fn from_set_row(
    spec: &SetRowSpec,
    names: &StageNames,
    float: bool,
    dense: bool,
    target: &SpatialTarget,
) -> Result<SpatialProgram, SpatialReject>

Set row index of T to S — the KV-cache append, as ONE rewrite of the carried table: T'[c] = (lo ≤ c < lo + cols) ? S[c − lo] : T[c].

★ AN OUT-OF-RANGE ROW MUST STILL FAULT, AND HERE NOTHING WOULD. The source is read through an IndirectTerm only where the row is selected; with lo out of range no position is, so the gather never runs and the write silently vanishes — where every tier fails. So the offset is CHECKED first by a gather of STRIDE ZERO (it reads lo itself, and its extent rows·cols is the range check): a bounds check is a gather that moves nothing.