feat(B.4b): WorldPicker.Pick — ray-sphere entity pick

Adds Pick(origin, direction, candidates, skipServerGuid, maxDistance)
to AcDream.Core.Selection.WorldPicker. Iterates candidates, skips
entities with ServerGuid==0 (atlas/dat-hydrated statics — no server
identity) and the caller's skipServerGuid (the player self).
Geometric ray-sphere intersection at 5m radius (matches
WorldEntity.DefaultAabbRadius). Returns the nearest hit's ServerGuid
within maxDistance (50m default), or null on miss.

6 xUnit tests added: hit, miss, two-in-line-returns-closer, skip-guid,
skip-zero-server-guid, beyond-max-distance.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Selection/WorldPicker.cs

@@ -1,3 +1,5 @@
+using System;
+using System.Collections.Generic;
 using System.Numerics;
 using AcDream.Core.World;
 
@@ -52,4 +54,55 @@ public static class WorldPicker
             return (Vector3.Zero, Vector3.Zero);
         return (nearWorld, Vector3.Normalize(dir));
     }
+
+    /// <summary>
+    /// Ray-sphere intersection against each candidate's <see cref="WorldEntity.Position"/>
+    /// using a fixed 5m sphere radius. Returns the <see cref="WorldEntity.ServerGuid"/>
+    /// of the closest hit within <paramref name="maxDistance"/>, or null on miss.
+    /// </summary>
+    /// <remarks>
+    /// Entities with <c>ServerGuid == 0</c> (atlas-tier scenery, dat-hydrated
+    /// statics) are skipped — they have no server-side identity and can't be
+    /// the target of a Use packet. The player's own guid is skipped via
+    /// <paramref name="skipServerGuid"/>.
+    /// </remarks>
+    public static uint? Pick(
+        Vector3 origin, Vector3 direction,
+        IEnumerable<WorldEntity> candidates,
+        uint skipServerGuid,
+        float maxDistance = 50f)
+    {
+        const float Radius  = 5f;
+        const float Radius2 = Radius * Radius;
+
+        if (direction.LengthSquared() < 1e-10f) return null;
+
+        uint? bestGuid = null;
+        float bestT    = float.PositiveInfinity;
+        foreach (var entity in candidates)
+        {
+            if (entity.ServerGuid == 0u)              continue;
+            if (entity.ServerGuid == skipServerGuid)  continue;
+
+            // Geometric ray-sphere: oc = origin - center, b = dot(oc, dir),
+            // c = |oc|^2 - r^2, discriminant = b^2 - c. If discriminant < 0
+            // the ray misses the sphere. Otherwise nearest intersection is
+            // t = -b - sqrt(discriminant).
+            var oc   = origin - entity.Position;
+            float b  = Vector3.Dot(oc, direction);
+            float c  = Vector3.Dot(oc, oc) - Radius2;
+            float d  = b * b - c;
+            if (d < 0f) continue;
+
+            float t = -b - MathF.Sqrt(d);
+            if (t < 0f)            continue;   // ray points away or origin inside
+            if (t >= maxDistance)  continue;
+            if (t < bestT)
+            {
+                bestT    = t;
+                bestGuid = entity.ServerGuid;
+            }
+        }
+        return bestGuid;
+    }
 }