Zone.PeriodAround is the one owner: round a body, the pull its well gives at the rail's distance
(GravityField.Acceleration); round a bodyless node, the ring law (strength of the orbits inside, plus
half of what shares the ring). Orbit.Period and AsteroidBelt.Periods are written once at zone build,
before placement; BeltTargets copies the belt's periods. A parentless orbit has no field and stands at
its phase. Fixtures lose OrbitPeriod and gain wells where their rails must move. Builds; the new
OrbitsFromFieldTests follow in the next commit.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
a4977cbe48