Weight UK BRMA assignment by census private-rented households

changelog.d/1087-uk-brma-census-private-rented-weights.fixed.md

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+UK BRMA assignment (`frs_brma`) now draws each benefit unit's Broad Rental Market Area in proportion to census private-rented households in its region and BRMA, using the bedroom band that matches its LHA category (A and B one bedroom, C two, D three, E four or more), instead of the row counts of the 2019-20 LHA list of rents, whose Scottish, Welsh and Northern Ireland lists are copies of English BRMAs' lists (uk-data#515). The counts come from England and Wales Census 2021, Scotland's Census 2022 and Northern Ireland Census 2021; Northern Ireland's census has no bedrooms question, so its counts weight every category. `brma_private_rented_households.json` replaces `brma_rent_counts.json` in the same `cells[region][LHA_category][brma]` shape, and `tools/build_uk_brma_household_table.py` rebuilds it from the households CSV that uk-data#516 adds (sha256 `fd40dae0…`, 936 rows) (microcosm#1087).