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.Core/Selection/RetailSelectionMesh.cs

@@ -0,0 +1,32 @@
+using System.Numerics;
+
+namespace AcDream.Core.Selection;
+
+/// <summary>
+/// Immutable CPU geometry used by retail's world-selection pass. One instance
+/// represents one GfxObj part: its exact drawing-BSP root sphere and its visual
+/// polygons in DAT order.
+/// </summary>
+public sealed record RetailSelectionMesh(
+    Vector3 SphereCenter,
+    float SphereRadius,
+    IReadOnlyList<RetailSelectionPolygon> Polygons);
+
+/// <summary>One visual polygon. Vertex order and one/two-sidedness are DAT-authored.</summary>
+public sealed record RetailSelectionPolygon(
+    IReadOnlyList<Vector3> Vertices,
+    bool SingleSided);
+
+/// <summary>One part which survived the normal world-render visibility traversal.</summary>
+public readonly record struct RetailSelectionPart(
+    uint ServerGuid,
+    int PartIndex,
+    Matrix4x4 LocalToWorld,
+    RetailSelectionMesh Mesh);
+
+/// <summary>Retail picker result, including which physics part supplied the hit.</summary>
+public readonly record struct RetailSelectionHit(
+    uint ServerGuid,
+    int PartIndex,
+    double Distance,
+    bool PolygonHit);