Add SSI takeup variable and apply in benefit calculation

- Add takes_up_ssi_if_eligible with default_value=True
- Apply takeup multiplier in ssi.py for microsimulation
- Follows new pattern from PR #7326 (no random() in country package)

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

Author: MaxGhenis

changelog_entry.yaml

@@ -2,5 +2,7 @@
   changes:
     added:
       - Add liquid asset input variables (bank_account_assets, stock_assets, bond_assets), ssi_countable_resources, and spm_unit_cash_assets aggregation
+      - Add takes_up_ssi_if_eligible variable for SSI takeup modeling
     changed:
       - SSI resource test now uses actual imputed assets instead of random pass rate
+      - SSI benefit now applies takeup in microsimulation

policyengine_us/variables/gov/ssa/ssi/ssi.py

@@ -30,8 +30,11 @@ def formula(person, period, parameters):
         individual_max = p.individual * MONTHS_IN_YEAR
         capped_benefit = min_(benefit, individual_max)
 
-        return where(
+        final_benefit = where(
             deeming_applies,
             capped_benefit,
             benefit,
         )
+
+        takes_up = person("takes_up_ssi_if_eligible", period)
+        return final_benefit * takes_up

policyengine_us/variables/gov/ssa/ssi/takes_up_ssi_if_eligible.py

@@ -0,0 +1,9 @@
+from policyengine_us.model_api import *
+
+
+class takes_up_ssi_if_eligible(Variable):
+    value_type = bool
+    entity = Person
+    label = "Takes up SSI if eligible"
+    definition_period = YEAR
+    default_value = True