Function render_whatif_sell 

pub fn render_whatif_sell(r: &SellReport, magi_caveat: bool) -> String

Render a what-if sell SellReport (task #43). Headlines the MARGINAL federal tax (the sale’s OWN effect — withhyp − baseline), then the §1(h) bracket + room, the effective rate (or n/a for a loss), the §1212 carryforward disclosure (delta-based — the this-year ordinary offset AND the amount carried, NEVER a hard-coded $3,000), and the §1411 NIIT delta with its sign. magi_caveat prints the ad-hoc-profile “MAGI assumed = ordinary income” note. Read-only; the vault is never touched.