feat(selection): port retail polygon picking and vivid marker

Replace the projected Setup-sphere rectangle and independent physics-wall ray with retail's render-coupled picker: only visible server-object parts participate, each exact drawing sphere broad-phases the camera-eye ray, and first-in-DAT-order visual polygon hits globally outrank sphere fallbacks.

Replace the devtools-only procedural triangles with the retained gameplay VividTargetIndicator using retail client-enum surfaces 1..4, radar-blip colorization, Setup selection-sphere framing, and the exact eight-pixel viewport clamp.

Release build succeeds with zero warnings and all 5,886 tests pass with five intentional skips.

Co-authored-by: OpenAI Codex <codex@openai.com>

src/AcDream.App/Rendering/Selection/RetailSelectionGeometryCache.cs

@@ -0,0 +1,76 @@
+using System.Numerics;
+using AcDream.Core.Selection;
+using DatReaderWriter;
+using DatReaderWriter.DBObjs;
+
+namespace AcDream.App.Rendering.Selection;
+
+/// <summary>
+/// Decodes the exact CPU geometry consumed by retail mouse selection. The broad
+/// sphere comes from the drawing-BSP root; polygons retain GfxObj DAT order.
+/// </summary>
+internal sealed class RetailSelectionGeometryCache
+{
+    private readonly DatCollection _dats;
+    private readonly object _datLock;
+    // Render-thread owned. Dictionary permits a cached null for a GfxObj which
+    // legitimately has no drawing BSP; ConcurrentDictionary does not.
+    private readonly Dictionary<uint, RetailSelectionMesh?> _cache = new();
+
+    public RetailSelectionGeometryCache(DatCollection dats, object datLock)
+    {
+        _dats = dats ?? throw new ArgumentNullException(nameof(dats));
+        _datLock = datLock ?? throw new ArgumentNullException(nameof(datLock));
+    }
+
+    public RetailSelectionMesh? Resolve(uint gfxObjId)
+    {
+        if (_cache.TryGetValue(gfxObjId, out var cached))
+            return cached;
+        RetailSelectionMesh? loaded = Load(gfxObjId);
+        _cache[gfxObjId] = loaded;
+        return loaded;
+    }
+
+    private RetailSelectionMesh? Load(uint gfxObjId)
+    {
+        GfxObj? gfx;
+        lock (_datLock)
+            gfx = _dats.Get<GfxObj>(gfxObjId);
+
+        var root = gfx?.DrawingBSP?.Root;
+        if (gfx is null || root is null || root.BoundingSphere.Radius <= 0f)
+            return null;
+
+        var polygons = new List<RetailSelectionPolygon>(gfx.Polygons.Count);
+        foreach (var entry in gfx.Polygons)
+        {
+            var source = entry.Value;
+            if (source.VertexIds.Count < 3)
+                continue;
+
+            var vertices = new Vector3[source.VertexIds.Count];
+            bool valid = true;
+            for (int i = 0; i < source.VertexIds.Count; i++)
+            {
+                if (!gfx.VertexArray.Vertices.TryGetValue((ushort)source.VertexIds[i], out var vertex))
+                {
+                    valid = false;
+                    break;
+                }
+                vertices[i] = vertex.Origin;
+            }
+            if (!valid)
+                continue;
+
+            polygons.Add(new RetailSelectionPolygon(
+                vertices,
+                SingleSided: (int)source.SidesType == 0));
+        }
+
+        return new RetailSelectionMesh(
+            root.BoundingSphere.Origin,
+            root.BoundingSphere.Radius,
+            polygons);
+    }
+}