Reserve trajectory state for balance-sheet primitives; death-basis rules on the exit transition #45

Design-stage reservation (cheap now, expensive later)

PolicyEngine/microcosm#320 + TheAxiomFoundation/axiom-rules-engine#76 move capital gains to a primitives-plus-rules architecture: populace carries asset-class positions and accrued-gain fractions; a realization model converts accrued to realized; the rules engine computes each country's legal gain concept. The static v1 (cross-sectional realization hazard calibrated to admin totals) ships independently of this repo — but its natural permanent home is a dynamics transition process, and two hooks are worth reserving in the trajectory design now:

1. Balance-sheet state on the trajectory. Asset positions and accrued gains are persistent state evolving under per-period transitions (returns, saving, realization). Realization is intrinsically intertemporal — accrual accumulates across periods and the behavioral margin is timing (lock-in, rate anticipation, retirement) — so in the trajectory design it is a per-period hazard over carried state, not a cross-sectional draw. Holding-period-dependent rules (US long/short; any taper/indexation regime) become computable natively from acquisition-to-disposal spans instead of an assumed share.

2. Death is a capital-gains event in both tax systems. US basis step-up (IRC §1014) and UK death uplift both fire at mortality — i.e., at this repo's exit transition. The exit event should be able to carry balance-sheet state and trigger the country's death-basis rule (an Axiom formula), which also opens the estate-tax interaction. This is the concrete coupling point between the mortality process and the tax system.

No implementation asked here yet — just that the trajectory state schema and the exit-transition interface don't foreclose (1) and (2). The capacity-ladder pattern applies: static hazard v1 (populace) → dynamics-native transition process v2, both refereed by the same pre-registration machinery this repo already uses.