CoastingTests: a ship launched below top speed keeps it at the play ThrustScale 0.1 and at 1

Reads only the ship's velocity after the zone steps; reintroducing drag below top speed fails both cases (50 -> 18.39 in 10 s).

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

tests/Aetheria.Shared.Tests/CoastingTests.cs

@@ -0,0 +1,48 @@
+using System;
+using CultMath;
+using Xunit;
+using static CultMath.math;
+using float3 = CultMath.float3;
+using Random = CultMath.Random;
+
+// The consumer's side of aetheria-gravity:ruling:no-drag-below-top-speed: a ship launched below its hull's top speed and
+// left alone keeps that speed, at the global thrust tuning the game ships with (GameplaySettings.ThrustScale 0.1) as well as
+// at the fixtures' 1. Reads only the ship's velocity after the zone steps: not Drag, DragScale or any field of the fix.
+// Kills: drag reintroduced below top speed, or any deceleration whose size depends on ThrustScale.
+public sealed class CoastingTests
+{
+    private const float PlayThrustScale = .1f; // GameplaySettings.ThrustScale's default, the value play runs at
+
+    private static float CoastedSpeed(float thrustScale, float launchSpeed, float seconds)
+    {
+        using var cache = RestoredHullsTests.OpenCatalog();
+        var hull = cache.GetByName<HullData>("Longinus");
+        var settings = RestoredHullsTests.Settings();
+        settings.ThrustScale = thrustScale;
+        var items = new ItemManager(1u, cache, new ProvenanceLedger(), settings, _ => { });
+        items.Random = new Random(1u);
+        var random = new Random(1);
+        var pack = new LoadoutGenerator(ref random, items, null, null, null, .5f).GenerateShipLoadout(candidate => candidate == hull);
+        Assert.NotNull(pack);
+        var zone = new Zone(items, new PlanetSettings(), new ZonePack(), new GalaxyZone { Name = "Test", Owner = null }, null);
+        var ship = (Ship) EntitySerializer.Unpack(items, zone, pack);
+        zone.Admit(ship, piloted: false);
+        ship.Position = float3.zero;
+        zone.Update(0f);
+
+        ship.MovementDirection = float2(0, 0);
+        ship.Velocity = float2(0, launchSpeed);
+        for (var i = 0; i < (int) (seconds * 60); i++) zone.Update(1f / 60f);
+        return length(ship.Velocity);
+    }
+
+    [Theory]
+    [InlineData(PlayThrustScale)]
+    [InlineData(1f)]
+    public void AShipLaunchedBelowTopSpeedKeepsItsSpeed(float thrustScale)
+    {
+        const float launch = 50f; // Longinus's top speed is 100
+        var speed = CoastedSpeed(thrustScale, launch, 10f);
+        Assert.True(speed >= launch - .01f, $"ThrustScale {thrustScale}: launched at {launch}, coasting 10 s left it at {speed}");
+    }
+}