Document SSI disability transition precedence

Author: MaxGhenis

policyengine_us/tests/policy/baseline/gov/ssa/ssi/is_ssi_disabled.yaml

@@ -94,3 +94,13 @@
     ssi_engaged_in_sga: false
   output:
     is_ssi_disabled: false
+
+- name: Data-provided SSI disability criteria wins during transition
+  period: 2024
+  input:
+    would_pass_ssa_disability_screen: true
+    meets_ssi_disability_criteria: false
+    is_disabled: false
+    ssi_engaged_in_sga: false
+  output:
+    is_ssi_disabled: false

policyengine_us/variables/gov/ssa/ssi/eligibility/status/is_ssi_disabled.py

@@ -10,6 +10,8 @@ class is_ssi_disabled(Variable):
     reference = "https://www.law.cornell.edu/uscode/text/42/1382c#a_3_A"
 
     def formula(person, period, parameters):
+        # Read the SSI compatibility variable so existing datasets that only
+        # store this input continue to override the broad fallback.
         meets_disability_criteria = person("meets_ssi_disability_criteria", period)
         engaged_in_sga = person("ssi_engaged_in_sga", period)
         return meets_disability_criteria & ~engaged_in_sga