mask_query.flag_positions treats every negative map value as off-coverage #914

shapepipe.utilities.mask_query.flag_positions zeroes every negative integer map value, not only healsparse's off-coverage sentinel, so a map that legitimately carried negative values would read as clean. Correct today (UNIONS maps carry non-negative bits), but the contract off-coverage-is-clean holds only by that accident; compare against the map's sentinel explicitly. Found while writing that contract (#875).