Fix unbound local variable in reform parameter application

When applying reform parameters with year-only period strings (e.g., "2027"),
the start and stop variables were not being set, causing an UnboundLocalError.

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

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

Author: MaxGhenis

policyengine_uk/parameters/gov/hmrc/student_loans/thresholds/plan_2.yaml

@@ -18,8 +18,29 @@ values:
     value: 28_470
     metadata:
       reference:
-        - title: Plan 2 threshold frozen until April 2027
-          href: https://www.gov.uk/government/publications/autumn-budget-2024-overview-of-tax-legislation-and-rates-ootlar/autumn-budget-2024-overview-of-tax-legislation-and-rates-ootlar
+        - title: Student loan thresholds for April 2025
+          href: https://www.gov.uk/government/publications/student-loan-and-postgraduate-loan-thresholds/student-loan-and-postgraduate-loan-thresholds-for-the-2025-to-2026-academic-year
+  2026-04-06:
+    value: 28_470
+    metadata:
+      uprating: none
+      reference:
+        - title: Autumn Budget 2025 - Plan 2 threshold freeze
+          href: https://obr.uk/efo/economic-and-fiscal-outlook-november-2025/
+  2027-04-06:
+    value: 28_470
+    metadata:
+      uprating: none
+      reference:
+        - title: Autumn Budget 2025 - Plan 2 threshold freeze
+          href: https://obr.uk/efo/economic-and-fiscal-outlook-november-2025/
+  2028-04-06:
+    value: 28_470
+    metadata:
+      uprating: none
+      reference:
+        - title: Autumn Budget 2025 - Plan 2 threshold freeze
+          href: https://obr.uk/efo/economic-and-fiscal-outlook-november-2025/
 metadata:
   label: Plan 2 threshold
   unit: currency-GBP

policyengine_uk/utils/scenario.py

@@ -125,6 +125,8 @@ def modifier(sim: Simulation):
                                 stop = instant(period_str.split(".")[1])
                                 period_ = None
                             else:
+                                start = None
+                                stop = None
                                 period_ = period(period_str)
                             sim.tax_benefit_system.parameters.get_child(
                                 parameter