Add reform impacts dashboard planning doc (#806)

[vahid-ahmadi]

Summary

• Add dashboard tracking reform impacts #806 asks for a dashboard tracking high-importance reform budgetary impacts over time and across model / data versions.
• Key audit finding: the fixtures already exist. tests/microsimulation/reforms_config.yaml has 9 canonical regression entries (basic-rate +1pp, UC taper to 20%, VAT +2pp, etc.) with expected impacts and tolerances, and test_parametric_reform_impacts.py exercises them on every PR. What's missing is a time series view onto them.
• New planning page at docs/book/validation/reform-impacts-dashboard-plan.md proposes:
  • append-only storage of (timestamp, version, dataset, reform_name, expected_impact, observed_impact, gap) rows on a CSV / Parquet (HuggingFace dataset or gh-pages artefact);
  • scheduled runner (weekly + on tag) walking reforms_config.yaml;
  • three publication paths in order of effort — docs page Plotly chart, living JSON endpoint, policyengine.org dashboard tile;
  • annotations for each known regime shift (EFO release, dataset refresh, methodology PR);
  • initial reform list = the existing 9 fixtures plus a roadmap for v2 (two-child limit removal, PA freeze extension, employer NI changes).
• Cross-links to the nowcasting comparison, RF methodology, and child-poverty validation pages so the four together function as a coherent "how is the model doing" view.
• Pure planning; no code or fixture changes.

Test plan

• Markdown lints cleanly (cross-refs, MyST admonition).
• Docs build job picks up the new page under validation/.

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