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/ZoneData.cs

@@ -92,9 +92,6 @@ public abstract class BodyData
 
     [JsonProperty("depthMul")] [Key(7)]
     public float GravityDepthMultiplier = 1;
-
-    [JsonProperty("depthExp")] [Key(8)]
-    public float GravityDepthExponent = 16;
 }
 
 [CultDocument("aetheria.planetdata", "1"), MessagePackObject, JsonObject(MemberSerialization.OptIn)]