compute_integration_windows: Compute integration window bounds from a boolean mask
Description
For `ff()` terms with limits (e.g., `limits = "s<t"`), computes the column
range containing TRUE values for each row of the mask.
Usage
compute_integration_windows(use_mask)
Value
Integer matrix with 3 columns: start index, end index, and width
for each row.
Arguments
- use_mask
Logical matrix indicating which (s,t) pairs are within the
integration limits.