[codex] Align SPM thresholds and stabilize clone-half priors

[MaxGhenis]

What changed

• align data-side SPM threshold helpers with the same reference-threshold and equivalence-scale logic used in policyengine-us
• make CD geographic adjustments tenure-specific instead of applying a renter-style adjustment to every cloned SPM unit
• replace the stale hardcoded housing target with a year-specific Census CPS ASEC SPM_CAPHOUSESUB benchmark for spm_unit_capped_housing_subsidy
• add a separate HUD USER benchmark path for modeled housing_assistance, so Census SPM capped subsidy and HUD spending/assisted-household counts are no longer mixed together
• stop stage-2 QRF from imputing spm_unit_spm_threshold for the PUF clone half
• rebuild clone-half spm_unit_spm_threshold deterministically from the donor half's geography and the current threshold formula
• replace the additive +1 sparse-reweighting prior with deterministic near-zero priors for zero-weight clone households, while keeping donor-half priors close to their survey weights
• add regression tests for future-year threshold reconstruction, tenure-specific CD GEOADJ values, clone-half threshold rebuilding, and sparse-prior initialization

Why

The data pipeline had three distinct SPM issues:

• threshold reconstruction in policyengine-us-data had drifted from the model-side logic in policyengine-us
• local-area recalculation reused a renter-style GEOADJ across owners and renters
• clone-half enhanced CPS generation was giving zero-weight synthetic households a meaningful starting prior and letting stage-2 QRF learn spm_unit_spm_threshold, even though thresholds should be derived from donor geography plus composition, not predicted statistically

The housing benchmark cleanup is separate but related concept hygiene:

• spm_unit_capped_housing_subsidy is a Census SPM concept and should be benchmarked to CPS ASEC SPM_CAPHOUSESUB
• housing_assistance is a HUD program/spending concept and should be benchmarked separately to HUD USER assisted-household counts and spending totals

Impact

• CPS-derived thresholds and local-area cloned thresholds now use the same future-year reference threshold path as policyengine-us
• CD-based local-area outputs no longer reuse one GEOADJ across owners and renters
• national housing calibration for the SPM capped subsidy now uses the Census concept instead of a stale mixed HUD/Census hardcoded value
• validation output now reports the Census capped-subsidy benchmark and HUD USER housing-assistance benchmark as separate rows
• clone-half enhanced CPS generation now starts zero-weight synthetic households near zero in the sparse optimizer instead of around weight 1
• clone-half SPM thresholds are rebuilt from donor geography instead of being stage-2 QRF outputs

Root cause

policyengine-us-data had:

• a separate threshold forecast path from policyengine-us
• one CD GEOADJ per district built with renter assumptions and reused for all tenures
• a housing benchmark path that mixed Census SPM capped subsidy and HUD spending concepts
• clone-half enhanced CPS generation that treated zero-weight synthetic households like ordinary weighted donors during sparse-prior initialization and allowed spm_unit_spm_threshold into the CPS-only QRF output set

Validation

• uv run pytest -q tests/unit/test_extended_cps.py
• uv run pytest -q tests/unit/calibration/test_calibration_puf_impute.py
• uv run pytest -q policyengine_us_data/tests/test_local_area_calibration/test_spm_thresholds.py
• uv run pytest -q tests/integration/test_enhanced_cps.py -k 'household_count or poverty_rate_reasonable'
• git diff --check

[MaxGhenis]

Superseded by #702. This draft was opened from a forked branch and is blocked by the workflow gate, so CI cannot run here.