fix(N.5b T6): index-length validation + document VertsPerLandblock %6 invariant

Code review (Important #1): AddLandblock validated Vertices.Length but
not Indices.Length. The indices loop indexes meshData.Indices[0..383]
unconditionally — out-of-range input would throw IndexOutOfRangeException
instead of the clearer ArgumentException the vertex check raises. Today
LandblockMesh.Build always produces 384/384, so this is defensive
forward-compat for future mesh sources.

Code review (Important #2): The shader (terrain_modern.vert:gl_VertexID
% 6) only correctly picks the cell-corner index because we bake
`slot * VertsPerLandblock` into indices and 384 is a multiple of 6.
That invariant is now documented in a comment near the constant — anyone
changing it must audit the shader.

Build green: 0 errors / 0 warnings.

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

src/AcDream.App/Rendering/TerrainModernRenderer.cs

@@ -17,7 +17,14 @@ namespace AcDream.App.Rendering;
 /// </summary>
 public sealed unsafe class TerrainModernRenderer : IDisposable
 {
-    private const int VertsPerLandblock = LandblockMesh.VerticesPerLandblock;  // 384
+    // VertsPerLandblock MUST stay divisible by 6 — terrain_modern.vert uses
+    // `gl_VertexID % 6` to pick the cell-corner index (BL/BR/TR/TL), and
+    // because we bake `slot * VertsPerLandblock` into indices CPU-side and
+    // pass BaseVertex=0 to MultiDrawElementsIndirect, gl_VertexID becomes
+    // `slot * VertsPerLandblock + local_index`. The shader's modulo-6 only
+    // reduces to `local_index % 6` because 384 is a multiple of 6. Changing
+    // either constant without auditing the shader will silently mis-render.
+    private const int VertsPerLandblock = LandblockMesh.VerticesPerLandblock;  // 384 (= 64 cells * 6 verts)
     private const int IndicesPerLandblock = VertsPerLandblock;
     private const int VertexSize = 40;  // sizeof(TerrainVertex)
     private const int IndexSize = sizeof(uint);
@@ -89,6 +96,10 @@ public sealed unsafe class TerrainModernRenderer : IDisposable
             throw new ArgumentException(
                 $"Expected {VertsPerLandblock} vertices, got {meshData.Vertices.Length}",
                 nameof(meshData));
+        if (meshData.Indices.Length != IndicesPerLandblock)
+            throw new ArgumentException(
+                $"Expected {IndicesPerLandblock} indices, got {meshData.Indices.Length}",
+                nameof(meshData));
 
         if (_idToSlot.ContainsKey(landblockId))
             RemoveLandblock(landblockId);