Set Marriage Allowance take-up rate to HMRC outturn (#368) (#369)

The placeholder 1.0 (100%) overstated Marriage Allowance take-up.
HMRC outturn shows ~2.1m claimants of ~4.2m eligible couples (~50%),
which means microsimulation overstates the program cost by ~£500m/year.
Set the parameter to 0.5 with a House of Commons Library citation.

Companion change in PolicyEngine/policyengine-uk#1636.
Source issue: PolicyEngine/policyengine-uk#623.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vahid-ahmadi

changelog.d/368.md

@@ -0,0 +1 @@
+- Set Marriage Allowance take-up rate to 0.5 (HMRC outturn ~2.1m claimants of ~4.2m eligible couples) instead of the placeholder 1.0, so microsimulation no longer overstates Marriage Allowance cost by ~£500m/year.

policyengine_uk_data/parameters/take_up/marriage_allowance.yaml

@@ -1,6 +1,9 @@
-description: Percentage of eligible couples who claim Marriage Allowance.
+description: Share of eligible couples who claim Marriage Allowance. HMRC outturn shows ~2.1m claimants vs ~4.2m eligible (~50% take-up). Read by `datasets/frs.py` to populate `would_claim_marriage_allowance` stochastically per person.
 metadata:
   unit: /1
   label: Marriage Allowance take-up rate
+  reference:
+    - title: House of Commons Library — Income tax allowances for married couples (SN00870)
+      href: https://commonslibrary.parliament.uk/research-briefings/sn00870/
 values:
-  2000-01-01: 1
+  2000-01-01: 0.5