Add Scottish Child Payment as calibration target (#261)

* Add Scottish Child Payment as calibration target

- Add SCP total spend to calibration targets
- Source: Scottish Budget 2026-27, Table 5.08
- Values: £455.8m (2024), £471.0m (2025), £484.8m (2026)
- Extrapolates 3% growth for other years

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

* Fix SCP calibration target ordering

Move scottish_child_payment df assignment to after the statistics lookup
to avoid KeyError when looking up targets. Custom targets (not in
statistics CSV) should be added after the initial targets lookup.

Also use household_from_person instead of household_from_family since
scottish_child_payment is now a Person-level variable.

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

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: MaxGhenis

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    added:
+      - Add Scottish Child Payment as a calibration target using Scottish Budget figures

policyengine_uk_data/utils/loss.py

@@ -519,6 +519,23 @@ def pe_count(*variables):
     target_names.append("dwp/pip_dl_enhanced_claimants")
     target_values.append(1_608_000)
 
+    # Scottish Child Payment total spend
+    # Source: Scottish Budget 2026-27, Table 5.08
+    # https://www.gov.scot/publications/scottish-budget-2026-2027/pages/6/
+    scp = sim.calculate("scottish_child_payment")
+    df["sss/scottish_child_payment"] = household_from_person(scp)
+    SCP_SPEND = {
+        2024: 455.8e6,
+        2025: 471.0e6,
+        2026: 484.8e6,
+    }
+    # Extrapolate for other years using 3% annual growth
+    scp_target = SCP_SPEND.get(
+        int(time_period), 471.0e6 * (1.03 ** (int(time_period) - 2025))
+    )
+    target_names.append("sss/scottish_child_payment")
+    target_values.append(scp_target)
+
     # Council Tax band counts
 
     ct_data = pd.read_csv(STORAGE_FOLDER / "council_tax_bands_2024.csv")