Implement Census-style housing subsidy capping

Census's SPM method distinguishes the estimated housing subsidy from the capped value included in SPM resources.

Current follow-up from PolicyEngine/policyengine-us#8327: keep `spm_unit_capped_housing_subsidy` as the canonical input variable for CPS ASEC `SPM_CapHouseSub`, because Census has already capped that public-use value. If we later simulate housing subsidies directly instead of reading the already-capped SPM value, we need to implement the Census cap rather than treating the full subsidy as a resource.

Census method from the SPM technical documentation:
- estimate housing subsidies as market rent minus total tenant payment, using a statistical match with HUD administrative data from PIC/TRACS for CPS ASEC households reporting rental help or public housing;
- estimate total tenant payment using HUD program rules, generally the greater of 30 percent of adjusted income or 10 percent of gross income;
- if a household has multiple SPM units, prorate the household subsidy by SPM-unit people divided by household people;
- cap the subsidy included in SPM resources at the housing portion of the appropriate SPM threshold minus total tenant payment.

References:
- Census SPM technical documentation, section 4.3.1 Housing subsidies: https://www2.census.gov/programs-surveys/supplemental-poverty-measure/datasets/spm/spm_techdoc.pdf
- IPUMS CPS SPMCAPHOUS description: https://cps.ipums.org/cps-action/variables/SPMCAPHOUS
- 2024 CPS ASEC public data dictionary for `SPM_CapHouseSub`: https://www2.census.gov/programs-surveys/cps/datasets/2024/march/asec2024_ddl_pub_full.pdf