Update the ISA cash limit and the non-cash ISA charge when the November 2026 ISA instrument is laid #2728

Budget 2026 retains the £20,000 cash ISA limit for all savers from April 2027. The secondary legislation is "to be laid before parliament in November 2026" (OOTLAR para 2.5).

Until it is laid, SI 2026/1018 is the law from 6 April 2027:

• reg 6 inserts reg 4ZA(1ZA), a "£12,000" cash limit for a person who is "64 or under at the end of that year";
• reg 15 inserts reg 22A, the final charge on cash interest in non-cash ISAs;
• it also restricts transfers into cash ISAs.

#2712 follows the Budget, not SI 2026/1018:

• gov.hmrc.isa.cash.subscription_limit gives cash no limit of its own from 2027. Its only reference for that is OOTLAR para 2.5.
• gov.hmrc.isa.cash_interest_charge.in_effect stays true from 6 April 2027, since the Budget documents don't say whether the charge survives.

When the November instrument is laid:

1. Cash limit. Check whether it revokes reg 4ZA(1ZA) or amends it.
   • Replace the OOTLAR reference in gov/hmrc/isa/cash/subscription_limit.yaml and full_limit_age.yaml with the instrument.
   • If it sets a different limit or age instead, change the values and the YAML tests.
2. Cash-interest charge. Check whether it revokes reg 15 (reg 22A).
   • If it does, end gov.hmrc.isa.cash_interest_charge.in_effect at 6 April 2027.
   • Update isa_cash_interest_flat_rate_charge, its tests and the ISA assumptions page.
3. Transfers. Note what happens to the restrictions on transfers into cash ISAs. The model doesn't cover transfers.
4. Axiom. Update Encode ISA return exemptions, Lifetime ISA bonuses and withdrawals, and the 2027 noncash interest charge TheAxiomFoundation/rulespec-uk#487. Its follow-up holds the reg 4ZA(1ZA) cases until this instrument.

Raised in Pavel's review of #2712 (should fix 1). Related: #2710 and #2719.