refactor(runtime): own projectile simulation

Move projectile component identity, prediction invalidation, spatial worksets, authoritative corrections, and the retail physics step into AcDream.Runtime. Keep App as the DAT-shape and presentation adapter so ACE outcomes and visible behavior remain unchanged.

src/AcDream.Runtime/Physics/RuntimeProjectile.cs

@@ -0,0 +1,40 @@
+using AcDream.Core.Physics;
+
+namespace AcDream.Runtime.Physics;
+
+/// <summary>
+/// Presentation-free projectile component attached to one canonical Runtime
+/// entity incarnation. The body is retail's one <c>CPhysicsObj</c>; the
+/// prepared Setup sphere and prediction version die with the same incarnation.
+/// </summary>
+public interface IRuntimeProjectile
+{
+    PhysicsBody Body { get; }
+    ProjectileCollisionSphere CollisionSphere { get; }
+    ulong PredictionAuthorityVersion { get; }
+}
+
+internal sealed class RuntimeProjectile : IRuntimeProjectile
+{
+    internal RuntimeProjectile(
+        PhysicsBody body,
+        ProjectileCollisionSphere collisionSphere)
+    {
+        Body = body ?? throw new ArgumentNullException(nameof(body));
+        if (!collisionSphere.IsValid)
+        {
+            throw new ArgumentOutOfRangeException(
+                nameof(collisionSphere),
+                "A Runtime projectile requires one valid prepared Setup sphere.");
+        }
+
+        CollisionSphere = collisionSphere;
+    }
+
+    public PhysicsBody Body { get; }
+    public ProjectileCollisionSphere CollisionSphere { get; }
+    public ulong PredictionAuthorityVersion { get; private set; }
+
+    internal void InvalidatePrediction() =>
+        PredictionAuthorityVersion++;
+}