Add twenty planning and validation docs pages

[vahid-ahmadi]

Combines 20 docs-only PRs into one, so main gets one release instead of twenty (each merge that adds a changelog fragment triggers a version bump and a PyPI release).

Each page was re-checked against current main today and corrected where it had gone stale. The fixes are on each original PR branch; this branch takes their final files unchanged. The diff touches only docs/book/ pages and changelog.d/*.added.md fragments, with no code, parameters or tests.

Original PR	Page	Issue
#1712	programs/gov/hmrc/firm-vat.md	#1320, #1333
#1714	programs/gov/cms/child-maintenance.md	#669
#1718	assumptions/mp30k-uk-switch-plan.md	#1692
#1719	policy/indirect-tax-effects-plan.md	#1114
#1720	assumptions/non-uk-benefit-receipt-plan.md	#842
#1721	programs/gov/hmrc/pension-tax-relief.md	#704
#1722	programs/gov/hmrc/pensions-comparison-stbm.md	#672
#1723	assumptions/uk-pipeline-alignment-plan.md	#1621
#1724	validation/child-poverty-tcl.md	#1398
#1725	programs/gov/dwp/pip-points-plan.md	#1069
#1726	programs/gov/dfe/regional-childcare-plan.md	#1008
#1727	policy/future-date-announcements.md	#636
#1728	assumptions/takeup-analysis-plan.md	#1354
#1729	validation/policyengine-py-household-impact.md	#1628
#1730	validation/ubuntu-vs-macos.md	#1209
#1731	assumptions/capital-gains-distribution-plan.md	#818, #817
#1732	validation/reform-impacts-dashboard-plan.md	#806
#1735	programs/gov/ofgem/warm-home-discount-plan.md	#502
#1736	programs/gov/dwp/cold-weather-payment-plan.md	#435
#1737	programs/gov/dwp/bereavement-support-plan.md	#466, #465

The linked issues stay open: these pages are plans and diagnoses, not the implementations.

Checks:

• ruff format --check . (0.17.0) is clean, including the Python blocks in the pages.
• towncrier build --draft picks up all 20 fragments.
• No myst.yml entries are needed, because the docs build picks pages up by folder pattern.
• Cross-page links that previously depended on merge order (Compare PolicyEngine UK pension treatment with STBM (#672) #1722 → Document pension tax relief mechanics (#704) #1721, Add UK pipeline alignment planning doc (#1621) #1723 → Add non-UK household benefit receipt planning doc (#842) #1720) all resolve in this branch.

axiom: n/a: documentation only, no rules or parameters change.

🤖 Generated with Claude Code

[juaristi22]

Reviewed d50ee85a9664791d6c617135fed999418762f6a9 and all 20 consolidated pages. I found no blocking model defect, but three plans need technical corrections before they guide implementation:

1. docs/book/assumptions/takeup-analysis-plan.md:95–97: restrict the numerator to eligible units too: sum(would_claim_X * eligible_X * weight) / sum(eligible_X * weight). Willingness-to-claim flags can be true for ineligible units; one eligible unit and two willing units currently gives a documented rate of 200%. Define the zero-denominator case as well.
2. docs/book/policy/future-date-announcements.md:67–71: restoring 20% on the reversal announcement date does not cancel an existing future 19% row. An executed check through fiscal-year conversion leaves 2023 at 19%. Remove or replace the cancelled row at its original effective date, retaining the announcement history in references/version control.
3. docs/book/programs/gov/dwp/bereavement-support-plan.md:72–84: a flag for bereavement within the previous 18 months can remain true across consecutive annual periods, so the proposed formula can award the initial lump sum twice. Use a payment/claim-year flag or event date for the single initial payment, and reserve the 18-month information for instalments.

Minor: docs/book/programs/gov/dwp/pip-points-plan.md:163 needs the closing ) on the DWP statistics link.

Validation: all 24 well-formed local links resolve; all six Python blocks parse; the dated-row diagnostic ran; 21 cited/primary sources checked. Captured documentation CI passes. I did not rerun the full documentation build or historical private-dataset benchmarks locally. The 20 older docs PRs can continue to be handled as superseded by this bundle.