Add bereavement support plan: BSP formula + historical WPA (#466 #465)

[vahid-ahmadi]

Summary

• Formula for Bereavement Support Payment #466 asks for a rule-based Bereavement Support Payment formula (the current bsp.py is just adds = [\"bsp_reported\"] — no derived eligibility). Widowed Parent's Allowance #465 asks for historical Widowed Parent's Allowance for back-cast analysis.
• These are the same pair of questions, so persist them in one combined plan at docs/book/programs/gov/dwp/bereavement-support-plan.md.
• Covers:
  • the current reported-only state and what it can't do (reform analysis, post-FRS-reference bereavement, historical WPA);
  • BSP rule structure: lump sum + 18 monthly instalments at higher (with dependent child) or standard rate; cohabiting-partner extension from 9 Feb 2023 via SI 2023/134;
  • Phase 1: BSP rule-based formula with three new input variables (is_bereaved_recently, bsp_higher_rate_eligible, bsp_months_remaining);
  • Phase 2: WPA (historical) with the same formula shape but per-year uprated rate and the 2001-04 to 2017-04 bereavement-date window;
  • Phase 3: data-side is_bereaved_recently flag imputation in policyengine-uk-data from FRS marital-status transitions and DWP bereavement statistics;
  • open questions on BSP uprating (currently flat since 2017), a reform-side "treat WPA as BSP" toggle, and the cohabiting-partner effective-date logic.
• Closes both Formula for Bereavement Support Payment #466 and Widowed Parent's Allowance #465 in the planning sense.
• Pure documentation; no code changes.

Test plan

• Markdown lints cleanly.
• Docs build job picks up the new page under programs/gov/dwp/.

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