Replace or demote the spm_unit_capped_housing_subsidy calibration target

[MaxGhenis]

spm_unit_capped_housing_subsidy is currently calibrated to a HUD/Census hybrid national total. If we want a credible hard target, this should be replaced with a clearer HUD administrative benchmark or demoted to diagnostic status.

Why the current target is weak:

• The current target is not clearly a single administrative source.
• HUD administrative program totals and assisted-housing records are more authoritative than a HUD/Census hybrid.
• The SPM capped housing subsidy concept is not identical to HUD outlays or administrative payment totals, so concept mapping needs to be explicit.

Proposed change:

• Either replace the current target with a HUD administrative rental-assistance benchmark, or demote the target to diagnostic status if the concept match is too loose.
• Candidate sources include HUD Picture of Subsidized Households and HUD rental-assistance budget/outlay materials.

Primary sources:

• HUD assisted housing datasets: https://www.huduser.gov/portal/datasets/assthsg.html
• HUD Picture of Subsidized Households dictionary: https://www.huduser.gov/portal/datasets/pictures/dictionary_2023.pdf
• HUD rental assistance overview: https://www.hud.gov/sites/dfiles/CFO/documents/2024_Rental_Assistance_Overview.pdf