Build a fired leg's context: the stored NON-reserved keys UNDER the
runtime-derived context. Stored values are spread first and runtime-derived
values last so the runtime value wins on any collision — the same
last-spread-wins ordering
#requestContextFor uses (runtime.ts:701). After stripping, the stored keys
share no key with the runtime-derived set (the reserved set IS the
runtime-owned set), so the order is defense-in-depth: a reserved key that ever
slipped the strip still loses. A pure helper a host applies when it seeds a
fired run's context from the tick-provided stored context.
Build a fired leg's context: the stored NON-reserved keys UNDER the runtime-derived context. Stored values are spread first and runtime-derived values last so the runtime value wins on any collision — the same last-spread-wins ordering #requestContextFor uses (runtime.ts:701). After stripping, the stored keys share no key with the runtime-derived set (the reserved set IS the runtime-owned set), so the order is defense-in-depth: a reserved key that ever slipped the strip still loses. A pure helper a host applies when it seeds a fired run's context from the tick-provided stored context.