Document that Northern Ireland business rates count regional and district rates

[MaxGhenis]

Fixes #2343

Summary

Documentation only. No values or formulas change. It explains why business_rates.statistics.revenue.NORTHERN_IRELAND (£741m in 2024-25) is about twice the OBR line that projects it (£382m):

• The NI series counts the whole rate bill: the regional rate plus each council's district rate, levied as one bill (Rates (Northern Ireland) Order 1977, art 9(1)).
• The OBR's "Northern Ireland business rates" line (receipts Table 3.2) covers the regional rate only.

Stacked on #2322 (base update-corporate-receipts-statistics). Merge it into that branch before #2322 merges, or cherry-pick the commit.

Evidence

All figures are £m, for 2024-25:

	Value	Source
LPS non-domestic rates paid (the statistic)	741.3	OpenDataNI, "Non-Domestic Rates collected by District Council", row "Northern Ireland"
Regional part, computed	363.7	Each council's payments × regional/(regional + district) poundage. Regional 29.02p (NISR 2024/28 art 3(1)); district 25.57p to 38.22p (DfC Rate Statistics 2025/26, Table 4)
District part, computed	377.7	Remainder
ONS, NI regional non-domestic rates (central government)	327	ONS CRPSF revenue tables, FYE 2025, sheet "Northern Ireland", row 38
ONS, NI district non-domestic rates (local government)	378	Same table, row 47
OBR, "Northern Ireland business rates"	382.3	OBR March 2026 receipts, Table 3.2, cell C7

DoF corroborates the split:

• Regional rate income is "forecasted to generate just over £900m" in 2026-27 (NI Assembly written statement, 12 February 2026).
• The OBR's two NI lines (domestic and business) sum to £913.7m that year.

Decision

• Keep the level. Counting only the regional rate would halve the rates NI businesses pay. It would also break symmetry with:
  • domestic_rates, which charges the full NI bill;
  • the GB series, which count all non-domestic rates paid, including the shares councils retain.
• Keep the index. The OBR line is the only official forecast. Applying it to the district part is now stated as an assumption.
  • The alternative uses enacted 2026-27 poundages: regional +3.01%, district +3.75%, combined +3.40%, against the OBR's +2.01%.
  • That would add £10.7m to 2026-27, which is immaterial, and poundage growth omits base changes.

Changes

• parameters/gov/hmrc/business_rates/statistics.yaml: header and NORTHERN_IRELAND comment and description.
• parameters/gov/economic_assumptions/yoy_growth.yaml: description of obr.business_rates.northern_ireland.
• data/fiscal_partition.yaml: the NI sentence of the business_rates data note.

Tests

test_fiscal_partition.py and test_corporate_receipts_statistics.py: 180 passed locally (uv run --frozen --extra dev pytest, head of this branch). No test reads the changed text. CI runs only for PRs into main.

Review

Independent review: pending.

axiom: n/a: documentation of official receipts statistics; no law encoded or changed

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