Ship contact is a simulation rule: Zone steps Contacts, Unity physics writes no Entity

Contacts.Step exchanges normal velocity between touching hull discs (PostCollisionVelocity moved verbatim
from ShieldManager) and publishes Zone.Contacted; ShieldManager presents it; HullCollider.OnCollisionEnter
and ShieldManager's ship-on-ship branch are deleted. Not yet built or tested (verification follows).

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

Assets/Scripts/ServerShared/Contacts.cs

@@ -0,0 +1,83 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+using System.Linq;
+using CultMath;
+using static CultMath.math;
+using float2 = CultMath.float2;
+
+// A hull touching a hull (aetheria-release cut sim-contact; ruling unity-presents-only-r3): the simulation owns
+// contact between entities. Each hull is a disc about its centre of mass that bounds every occupied cell, and
+// two discs that touch while closing along the line between their centres exchange their normal velocity.
+// Presentations read Zone.Contacted; no Unity collision callback writes an Entity.
+public readonly struct ContactEvent
+{
+    public readonly Entity A;
+    public readonly Entity B;
+    public readonly float2 Point;
+    // Unit vector from A to B.
+    public readonly float2 Normal;
+
+    public ContactEvent(Entity a, Entity b, float2 point, float2 normal)
+    {
+        A = a;
+        B = b;
+        Point = point;
+        Normal = normal;
+    }
+}
+
+public static class Contacts
+{
+    // The farthest corner of any occupied hull cell from the centre of mass the schematic frame uses, in world
+    // units: a true bound on how far the hull reaches from Position (the half-diagonal of the bounding box is not,
+    // as FireControl.Detonate's comment records). A cell at (x, y) spans x +/- .5 and y +/- .5.
+    public static float Radius(Entity e)
+    {
+        var shape = e.HullData.Shape;
+        var com = shape.CenterOfMass;
+        var far = 0f;
+        foreach (var c in shape.Coordinates)
+            foreach (var (dx, dy) in new[] { (-.5f, -.5f), (.5f, -.5f), (-.5f, .5f), (.5f, .5f) })
+                far = max(far, length(float2(c.x + dx, c.y + dy) - com));
+        return far * e.ItemManager.GameplaySettings.SchematicCellSize;
+    }
+
+    // The elastic exchange of one body's normal velocity: v1 and m1 the body's own, v2 and m2 the other's.
+    private static float PostCollisionVelocity(float v1, float m1, float v2, float m2)
+    {
+        return (v1 * (m1 - m2) + m2 * v2) / (m1 + m2);
+    }
+
+    // For each unordered pair of the zone's entities, in list order: if their discs touch and are closing along
+    // the normal, each one's normal velocity is replaced (both from the velocities the pair had on entering) and
+    // its tangent component kept, and the contact is published. A docked entity (it has a Parent and takes its
+    // parent's pose) and an inactive one are not hulls in the zone. No positional push-out.
+    public static void Step(Zone zone)
+    {
+        var entities = zone.Entities.Where(e => e.Active && e.Parent == null).ToArray();
+        var radii = entities.Select(Radius).ToArray();
+        for (var i = 0; i < entities.Length; i++)
+            for (var j = i + 1; j < entities.Length; j++)
+            {
+                var a = entities[i];
+                var b = entities[j];
+                var offset = b.Position.xz - a.Position.xz;
+                var distance = length(offset);
+                // Coincident centres have no normal.
+                if (distance <= 0f || distance >= radii[i] + radii[j]) continue;
+                var n = offset / distance;
+                var va = dot(n, a.Velocity);
+                var vb = dot(n, b.Velocity);
+                if (vb - va >= 0f) continue;
+
+                var tangent = float2(-n.y, n.x);
+                var vaNew = PostCollisionVelocity(va, a.Mass, vb, b.Mass);
+                var vbNew = PostCollisionVelocity(vb, b.Mass, va, a.Mass);
+                a.Velocity = n * vaNew + tangent * dot(tangent, a.Velocity);
+                b.Velocity = n * vbNew + tangent * dot(tangent, b.Velocity);
+                zone.Contacted.OnNext(new ContactEvent(a, b, a.Position.xz + n * radii[i], n));
+            }
+    }
+}

Assets/Scripts/ServerShared/Contacts.cs.meta

@@ -0,0 +1,11 @@
+fileFormatVersion: 2
+guid: 7c1e5a3b9d4f4e2a8b6c0d1f2e3a4b5c
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData:
+  assetBundleName:
+  assetBundleVariant:

Assets/Scripts/ServerShared/Zone.cs

@@ -54,6 +54,8 @@ public class Zone
     public List<PendingShot> PendingShots = new List<PendingShot>();
     public Subject<ShotOutcome> ShotCommitted = new Subject<ShotOutcome>();
     public Subject<ShotOutcome> ShotResolved = new Subject<ShotOutcome>();
+    // Contacts.Step publishes each hull-on-hull contact it resolves; presentations are the only readers.
+    public Subject<ContactEvent> Contacted = new Subject<ContactEvent>();
     private int _nextShotId;
     public int NextShotId() => ++_nextShotId;
 
@@ -241,6 +243,10 @@ public class Zone
         StepMines(deltaTime);
         StepClouds(deltaTime);
 
+        // Hull contact (cut sim-contact): after every entity has moved and the mines and clouds have taken
+        // their toll, so only hulls still in the zone touch, and before shots resolve against the result.
+        Contacts.Step(this);
+
         // Cut 3: after every entity has had its chance to fire this tick, age and resolve the shots that
         // firing queued. A shot fired this tick with a flight time shorter than CommitHorizon commits and
         // resolves in this same call.