Use actual imputed assets for SSI resource test

Remove microsimulation bypass that used random pass rate. Now that
liquid assets are imputed from SIPP, the resource test can use real
calculated values.

- Remove pass_rate.yaml parameter (no longer needed)
- Update README to reflect new methodology
- meets_ssi_resource_test now always uses ssi_countable_resources

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MaxGhenis

changelog_entry.yaml

@@ -2,3 +2,5 @@
   changes:
     added:
       - Add liquid asset input variables (bank_account_assets, stock_assets, bond_assets), ssi_countable_resources, and spm_unit_cash_assets aggregation
+    changed:
+      - SSI resource test now uses actual imputed assets instead of random pass rate

policyengine_us/parameters/gov/ssa/ssi/eligibility/resources/README.md

@@ -1,10 +1,16 @@
 # Resources
 
-[SSA reported $5.293 billion in SSI expenditures in February 2023](https://www.ssa.gov/policy/docs/quickfacts/stat_snapshot/#table3).
-Adjust `pass_rate.yaml` to match this for SSI in 2023 ($5.293 billion * 12 = $63.5 billion).
+SSI countable resources are now calculated from imputed liquid assets
+(bank accounts, stocks, bonds) rather than using a random pass rate.
+
+The assets are imputed from SIPP in policyengine-us-data and flow through
+to the SSI resource test via `ssi_countable_resources`.
 
-Check with this code:
+Check SSI expenditures with:
 ```python
 from policyengine_us import Microsimulation
 Microsimulation().calc("ssi", map_to="person", period=2023).sum() / 1e9
 ```
+
+[SSA reported $5.293 billion in SSI expenditures in February 2023](https://www.ssa.gov/policy/docs/quickfacts/stat_snapshot/#table3).
+Annual target: $5.293 billion * 12 = $63.5 billion.

policyengine_us/parameters/gov/ssa/ssi/eligibility/resources/pass_rate.yaml

@@ -5,8 +5,8 @@ class meets_ssi_resource_test(Variable):
     value_type = bool
     entity = Person
     label = "Meets SSI resource test"
-    unit = USD
     definition_period = YEAR
+    reference = "https://secure.ssa.gov/poms.nsf/lnx/0501110000"
 
     def formula(person, period, parameters):
         p = parameters(period).gov.ssa.ssi