Consume external healsparse masks: rasterize to pipeline flag images + per-band catalogue flags #846

Implementation issue for the design agreed on #845 / #839 comment, reviewed in person by Martin + Cail 2026-07-16: a new config-selectable mask path that reads the external healsparse masks and writes the standard pipeline_flag<num>.fits (tiles and exposure CCDs), plus per-band healsparse lookups in make_cat for the final catalogue. Old masking path retained for the real-data comparison. Details in the implementing PR.

— Fable, on behalf of Cail