refactor(A.5 T15): TerrainModernRenderer.AddLandblockWithMesh entry point

T13 routes worker-built meshes from LandblockStreamResult.Loaded.MeshData
into the renderer. AddLandblockWithMesh accepts a prebuilt mesh + origin
and delegates to the existing AddLandblock(uint, LandblockMeshData, Vector3)
so both paths share one upload path (Approach B -- AddLandblock already
takes a prebuilt mesh; no inline build to extract).

GameWindow's T16 lambda captures liveCenterX/Y and passes the derived
origin; the renderer stays origin-agnostic.

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

src/AcDream.App/Rendering/TerrainModernRenderer.cs

@@ -89,6 +89,18 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
         _indirectBuffer = _gl.GenBuffer();
     }
 
+    /// <summary>
+    /// Two-tier streaming entry point. Accepts a prebuilt mesh from
+    /// <see cref="LandblockStreamResult.Loaded.MeshData"/> built on the worker
+    /// thread, together with the world-space origin computed by the caller
+    /// (render-thread GameWindow derives it from landblockId + liveCenterX/Y).
+    ///
+    /// Delegates to <see cref="AddLandblock(uint,LandblockMeshData,Vector3)"/>
+    /// so both paths share one upload path. Per Phase A.5 spec T15.
+    /// </summary>
+    public void AddLandblockWithMesh(uint landblockId, LandblockMeshData meshData, Vector3 worldOrigin)
+        => AddLandblock(landblockId, meshData, worldOrigin);
+
     public void AddLandblock(uint landblockId, LandblockMeshData meshData, Vector3 worldOrigin)
     {
         ArgumentNullException.ThrowIfNull(meshData);