Replaces the flat 47% (45 PAYE + 2 NI) RSU marginal across panels 3, 7, 8, 11,
and 12 with an exact piecewise band-aware computation. Each row computes
ani_prior/ani_pre/ani_post over the tax-year YTD (chronological model — the
RSU is taxed at the band its YTD ANI position occupies at the vest date,
mirroring PAYE withholding behaviour).
Bands (2024/25+, applied to all years):
IT: 0% / 20% / 40% / 60% (PA-taper) / 45% at 12,570 / 50,270 / 100k / 125,140
NI: 0% / 8% / 2% at 12,570 / 50,270
PA-taper modelled as 60% effective IT marginal in £100k–£125,140
(40% on the £1 + 40% on the £0.50 of lost PA = 60%).
Spot-checked per tax-year totals via psql; numbers diverge from the flat
47% baseline most for years where vests cross PA-taper or basic-rate bands
(2020/21 ~35%, 2024/25 ~41%, 2025/26 ~43%).
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