feat(data): default Gini source moves to World Bank WDI

[setupelz]

Summary

• WIID (CC BY-NC-SA) becomes opt-in; WDI SI.POV.GINI (CC BY 4.0) is the default, latest observation in 2015-2023 per country
• analysis-country membership no longer depends on Gini availability: set grows 167 -> 176, missing Gini gets the analysis mean (gini_missing_policy: fallback-mean | strict), flagged gini_imputed
• eight duplicated coverage/Gini implementations routed through one helper; API path previously wrote country_gini_stationary.csv without the ROW row the notebook path had
• fix capability snapshots: the Gini adjustment now applies on the pre-window capability_reference_year branch too, budgets and pathways alike (was silently unadjusted, and documented as such)
• re-pin WDI GDP hashes to the regenerated 2026-07-13 upstream export
• analysis/gini_source_comparison.py quantifies the swap: 150 vs 194 countries, value drift is the consumption-vs-income welfare gap (largest: India +55 Gt on gini-adjusted budgets)
• fix notebook 100: passed the removed project_root= argument, RCB pipeline could not run

Lint & Test Results

• Lint: pre-existing repo-wide debt (118 errors on main baseline vs 116 on this branch); not introduced by this changeset, CI does not run ruff for this repo
• Tests: 742 passed, 11 deselected

Test Plan

• uv run python -m pytest tests/ -q