fix: state pension respects recorded amount for new-SP cohort

changelog.d/fixed/state-pension-recorded-amount.md

@@ -0,0 +1 @@
+Fix `person_state_pension` to respect the recorded `state_pension` amount for the new-SP cohort (those below the basic-SP age cutoff). Previously the new-SP branch always returned the full `new_state_pension_weekly × 52`, ignoring any recorded amount and over-stating SP for partial-year and partial-record claimants. Now mirrors the existing old-SP scaling pattern: recorded amounts pass through (scaled by reform-ratio when the new-SP rate changes), with fallback to the parameter rate when no amount is recorded. Closes #59. Shrinks the parity-harness pensioner-couple state-pension diff from £946 → £0.

src/engine/simulation.rs

@@ -112,6 +112,8 @@ pub struct Simulation {
     pub parameters: Parameters,
     /// Baseline old basic SP weekly rate for scaling reported amounts under reforms.
     pub baseline_old_sp_weekly: f64,
+    /// Baseline new SP weekly rate for scaling reported amounts under reforms.
+    pub baseline_new_sp_weekly: f64,
     /// Fiscal year (e.g. 2025 for 2025/26) — used for new/basic SP cutoff.
     pub fiscal_year: u32,
 }
@@ -125,25 +127,27 @@ impl Simulation {
         fiscal_year: u32,
     ) -> Self {
         let baseline_old_sp_weekly = parameters.state_pension.old_basic_pension_weekly;
+        let baseline_new_sp_weekly = parameters.state_pension.new_state_pension_weekly;
         Simulation {
             people, benunits, households, parameters,
-            baseline_old_sp_weekly, fiscal_year,
+            baseline_old_sp_weekly, baseline_new_sp_weekly, fiscal_year,
         }
     }
 
-    /// Create a simulation with explicit baseline old SP rate (for reform simulations
-    /// where the baseline rate differs from the reform parameters).
+    /// Create a simulation with explicit baseline SP rates (for reform simulations
+    /// where the baseline rates differ from the reform parameters).
     pub fn new_with_baseline_sp(
         people: Vec<Person>,
         benunits: Vec<BenUnit>,
         households: Vec<Household>,
         parameters: Parameters,
         baseline_old_sp_weekly: f64,
+        baseline_new_sp_weekly: f64,
         fiscal_year: u32,
     ) -> Self {
         Simulation {
             people, benunits, households, parameters,
-            baseline_old_sp_weekly, fiscal_year,
+            baseline_old_sp_weekly, baseline_new_sp_weekly, fiscal_year,
         }
     }
 
@@ -157,10 +161,11 @@ impl Simulation {
         // Phase 1a: Calculate each person's state pension under the current policy.
         // State pension is taxable income so must be computed before income tax.
         let baseline_old_sp = self.baseline_old_sp_weekly;
+        let baseline_new_sp = self.baseline_new_sp_weekly;
         let fiscal_year = self.fiscal_year;
         let person_sp: Vec<f64> = self.people.par_iter().map(|p| {
             variables::benefits::person_state_pension(
-                p, &self.parameters, baseline_old_sp, fiscal_year,
+                p, &self.parameters, baseline_old_sp, baseline_new_sp, fiscal_year,
             )
         }).collect();
 
@@ -184,7 +189,7 @@ impl Simulation {
             let hh = &self.households[bu.household_id];
             variables::benefits::calculate_benunit(
                 bu, &self.people, &person_results, hh, &self.parameters,
-                baseline_old_sp, fiscal_year,
+                baseline_old_sp, baseline_new_sp, fiscal_year,
             )
         }).collect();
         benunit_results = br;
@@ -708,6 +713,7 @@ mod tests {
             adjusted_people, benunits.clone(), households.clone(),
             policy_params.clone(),
             baseline_params.state_pension.old_basic_pension_weekly,
+            baseline_params.state_pension.new_state_pension_weekly,
             2025,
         );
         let dynamic_results = dynamic_sim.run();

src/main.rs

@@ -561,13 +561,14 @@ fn main() -> anyhow::Result<()> {
         dataset.people.clone()
     };
 
-    // Run policy simulation (pass baseline old SP rate so reported amounts scale correctly)
+    // Run policy simulation (pass baseline old + new SP rates so reported amounts scale correctly)
     let policy_sim = Simulation::new_with_baseline_sp(
         policy_people,
         dataset.benunits.clone(),
         dataset.households.clone(),
         policy_params.clone(),
         baseline_params.state_pension.old_basic_pension_weekly,
+        baseline_params.state_pension.new_state_pension_weekly,
         cli.year,
     );
     let reformed = policy_sim.run();