Computes lagged values within each cross-sectional unit. Returns NA for
the first k observations of each unit to avoid cross-unit contamination.
.lag_panel(panel, vars, k = 1L)A data.frame (or vector if single var) of lagged values, same
length as nrow(panel).
A panel data.frame from .make_panel().
Character vector of column names to lag.
Integer lag order (positive = lag, negative = lead).