Add Council Tax Reduction / Council Tax Benefit (CTR/CTB)

[vahid-ahmadi]

Addresses #42.

Council tax liability (band × LA-multiplier with the 25% single-person discount) was already modelled in src/variables/wealth_taxes.rs and surfaced as HouseholdResult.council_tax_calculated. This PR fills the remaining gap from the issue: the means-tested Council Tax Reduction / Council Tax Benefit (CTR/CTB) that reduces that liability and flows through to net income.

Scope

• New module src/variables/council_tax.rs — calculate_council_tax_reduction, the means test mirroring Housing Benefit (weekly applicable amount vs. income, excess tapered) but using the CTR/CTB 20% taper (SI 2012/2885 Sch.1 para.30) rather than HB's 65%.
• New CouncilTaxReductionParams (src/parameters/mod.rs): taper rate, working-age maximum-support cap, and HB-style applicable-amount allowances. Pension-age claimants receive up to 100% support; working-age claimants are capped at a representative England-average 90% (max_support_working_age), reflecting that most billing authorities now require a minimum contribution.
• Wired into the simulation (src/engine/simulation.rs): CTR is computed against the calculated council tax liability for the household reference person's benefit unit, surfaced on the new HouseholdResult.council_tax_reduction, counted in total_benefits, and added to HBAI net income — consistent with how other benefits flow.
• Output / wrapper: new council_tax_reduction field in the main.rs ProgramBreakdown and the Python ProgramBreakdown; new CouncilTaxReductionParams Pydantic model + Parameters.council_tax_reduction for reform overlays, exported from the package.
• 2025/26 parameters set in parameters/2025_26.yaml (Local Government Finance Act 2012 s.10; SI 2012/2885).
• Citations added to LEGISLATIVE_REFERENCE.md (new section 15).
• Changelog fragment changelog.d/added/council-tax-reduction.md.

Tests

cargo build and cargo test both pass (195 tests, 0 failures). New unit tests in council_tax.rs:

• no liability → no reduction
• low-income working-age → capped full support (90% of bill)
• pension-age low-income → 100% support
• high-income → tapered to zero
• mid-income → exact partial taper

Remaining

• Devolved CTR variants (Scotland SSI 2012/303, Wales SI 2013/3029) use different applicable amounts and 100%-support defaults; the current implementation applies the England-average scheme nationally. Documented in LEGISLATIVE_REFERENCE.md as a follow-up slice.
• Disability/carer premiums in the CTR applicable amount, non-dependant deductions, and capital limits are not yet modelled.
• Historical energy / cost-of-living council-tax rebates (issue item 4) are out of scope for this slice.

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