feat(engine): onshore/offshore tax model (v0.6.0)

[fall-development-rob]

Summary

• Implement per-wrapper tax computation with residence x domicile x wrapper matrix per ADR-037 / CONTRACT-020
• Add TaxLotTracker class for FIFO/HIFO cost-basis lot accounting and CGT computation
• Add computeWrapperTax for 11 wrapper types: Taxable, US-Traditional-401k/IRA, US-Roth-401k/IRA, UK-SIPP, UK-ISA, UK-Onshore/Offshore-Bond, Offshore-Trust, Cayman-Exempt-Company
• Add computeRMD with IRS Uniform Lifetime Table (ages 72-120)
• Add WITHHOLDING_TREATY_TABLE for dividend withholding with override support
• Add UK non-dom remittance basis with £30k charge
• Integrate wrapper tax into runAdvancedProjection with tax_breakdown and rmd_amount on TimelineRow
• Extend Scenario with tax_residence, tax_domicile, remittance_basis_charge, cgt_method, withholding_overrides, fatca_reporting, crs_reporting
• Extend FinancialItem with wrapper and cost_basis
• All new fields optional with defaults; v0.5 scenarios produce byte-identical output
• Version bump 0.5.0 -> 0.6.0
• 200 tests pass (59 new assertions across 20 new test cases)

Test plan

• computeRMD returns correct divisor at ages 73, 80, 90
• computeWrapperTax for Taxable: CGT > 0 when gain exists
• computeWrapperTax for US-Roth: tax-free when qualified
• computeWrapperTax for US-Roth: income tax when non-qualified
• computeWrapperTax for UK-SIPP: 25% tax-free lump applied
• computeWrapperTax for UK-ISA: zero tax
• TaxLotTracker FIFO ordering correct
• TaxLotTracker HIFO ordering correct
• Backwards compat: v0.5 scenario (no wrappers) produces identical output
• Build + lint clean

🤖 Generated with claude-flow