Sensor: the pinger's swept set decides each crossing once; allies credit a published ping return inside the bistatic oval

Deletes the stateless interval crossing (_previousRadius, PreviousRadius, Crossing) and the allied-pinger scan over
other ships' Sensors. The pinging Sensor remembers the subjects its wavefront has reached (_reached), in the beam
or not, and a subject's first sweeping tick inside the radius is its one crossing. On it the pinger paints, credits
itself and posts a PingReturn to each ally; Entity publishes returns with paints (PublishPaints is now
PublishInbox) and the ally's own sensors credit them for one tick at the geometric mean range, inside
Rt x Rr < PingRange^2, through the geometric mean of both legs' obscuration (ruling ally-echo-bistatic-oval).
This commit builds and replaces the sensor the previous commit's tests were written against.

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

Assets/Scripts/ServerShared/Sensing.cs

@@ -40,6 +40,24 @@ public enum PaintMode
     Track
 }
 
+// An ally's ping crossed `Subject` at `IlluminatedRange` through `IlluminatedObscuration`; `MaxRange` is that
+// sensor's PingRange. Posted on the crossing tick and read for one tick by the receiving ship's own sensors.
+public readonly struct PingReturn
+{
+    public readonly Entity Subject;
+    public readonly float IlluminatedRange;
+    public readonly float IlluminatedObscuration;
+    public readonly float MaxRange;
+
+    public PingReturn(Entity subject, float illuminatedRange, float illuminatedObscuration, float maxRange)
+    {
+        Subject = subject;
+        IlluminatedRange = illuminatedRange;
+        IlluminatedObscuration = illuminatedObscuration;
+        MaxRange = maxRange;
+    }
+}
+
 public readonly struct PaintWarning
 {
     public readonly float2 Bearing;