Use subtree for p in plan_2_interest_rate

p = parameters(period).gov.hmrc.student_loans for cleaner code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: MaxGhenis

policyengine_uk/variables/gov/hmrc/student_loans/plan_2_interest_rate.py

@@ -17,21 +17,17 @@ class plan_2_interest_rate(Variable):
 
     def formula(person, period, parameters):
         income = person("adjusted_net_income", period)
-        p = parameters(period).gov
+        p = parameters(period).gov.hmrc.student_loans
+        rpi = parameters(period).gov.economic_assumptions.yoy_growth.obr.rpi
 
         # Per Regulation 21AB, lower interest threshold = repayment threshold
         # Below lower threshold: RPI only
         # Above upper threshold: RPI + 3%
         # Between: linear taper
         taper_fraction = np.clip(
-            (income - p.hmrc.student_loans.thresholds.plan_2)
-            / (
-                p.hmrc.student_loans.interest_rates.plan_2.upper_threshold
-                - p.hmrc.student_loans.thresholds.plan_2
-            ),
+            (income - p.thresholds.plan_2)
+            / (p.interest_rates.plan_2.upper_threshold - p.thresholds.plan_2),
             0,
             1,
         )
-        return p.economic_assumptions.yoy_growth.obr.rpi + (
-            p.hmrc.student_loans.interest_rates.plan_2.additional_rate * taper_fraction
-        )
+        return rpi + (p.interest_rates.plan_2.additional_rate * taper_fraction)