Gravity: one potential per zone owns acceleration and height

GravityField is the law of one well (logarithmic tail, softened core).
Zone.GetForce and GetHeight sum it over the bodies; GetNormal derives from
the force for ship tilt; Flow and Ship read GetForce. Well strength, core
radius and Roche limit are PlanetSettings curves of mass, height
exaggeration K one global. Deletes GravityAcceleration, GravityReferenceRate,
GravityStrength, the PowerPulse well, GetNormal's finite differences and
BodyData.GravityDepthExponent. Settings.asset carries the calibrated curves.
Tests: PotentialOwnerTests, the moon's moon placement, retired keys over every
BodyData type, a body-derived giant sweep. Not yet built or run (verify job
follows); this commit may not build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/Ship.cs

@@ -130,9 +130,8 @@ public class Ship : Entity
         
         Position.xz += Velocity * delta;
         
+        Velocity += Zone.GetForce(Position.xz) * delta;
         var normal = Zone.GetNormal(Position.xz);
-        var force = new float2(normal.x, normal.z);
-        Velocity += Zone.GravityAcceleration(force, Zone.Settings.GravityStrength) * delta;
         var shipRight = Direction.Rotate(ItemRotation.Clockwise);
         var forward = cross(float3(shipRight.x, 0, shipRight.y), normal);
         Rotation = quaternion.LookRotation(forward, normal);