Add non-UK household benefit receipt planning doc (#842)

[vahid-ahmadi]

Summary

• Account for non-UK household benefit receipt #842 flagged that PolicyEngine UK's calibration implicitly assumes all DWP/HMRC outturn lands on FRS households, but it doesn't — exported State Pension, care-home residents, EU-treaty Child Benefit residue, and administrative leakage all sit outside the FRS sampling frame, inflating PolicyEngine's per-household estimates.
• Persist the plan in-repo at docs/book/assumptions/non-uk-benefit-receipt-plan.md.
• Three-phase route:
  • Phase 1 — quantify the gap per programme using DWP "Benefits paid outside the UK" statistics, the F&E in the Benefit System release, and Stat-Xplore caseload breakdowns. Per-programme table sketched.
  • Phase 2 — subtract the out-of-scope share from policyengine-uk-data calibration targets via a new private_household_share field on programs.yaml rows (default 1.0).
  • Phase 3 (optional) — keep an additive correction for headline fiscal aggregates rather than synthesising overseas / institutional households, since synthesis would need strong unverifiable assumptions.
• Pure documentation; the implementation lives mostly in policyengine-uk-data — only the private_household_share field would be added to this repo.

Test plan

• Markdown lints cleanly (table, links).
• Docs build job picks up the new page under assumptions/.

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