Replace the 2027-30 Scottish water growth with WICS's Final Determination (due 29 October 2026) #2538

#2536 adds gov.economic_assumptions.yoy_growth.scottish_water.water_bills. Its 2027-28 to 2029-30 values (3.9%, 3.9% and 4.1%) come from the Water Industry Commission for Scotland's Draft Determination for 2027-33:

• the core cap of CPI + 2% in each of the first three years (para. 9.1.3);
• WICS's forecast of October CPI in the previous year (Table 68: 1.9%, 1.9% and 2.1%, from OBR's March 2026 forecast).

Later years hold the 2029-30 rate.

WICS publishes the Final Determination on 29 October 2026, and charges under it start on 1 April 2027 (https://wics.scot/latest/wics-publishes-2027-33-draft-determination).

To do once it is out

• Replace the 2027-28 to 2029-30 values with the final caps, using the CPI the Final Determination applies, and cite it.
• If it sets caps for 2030-31 to 2032-33, add them. In the draft, those years were CPI + 2% only if the West Central Bioresources conditions were met, CPI + 0% otherwise, and up to 0.3% more for uncertain costs.
• Update the table in docs/book/assumptions/water-bills.md.
• When Scottish Water announces its 2027-28 charges (usually in late January), replace 2027-28 with the announced increase.

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