feat(core): add LandblockMesh flat-terrain generator

Pure CPU mesh generator: takes a DatReaderWriter LandBlock DBObj and
produces 81 vertices + 128 triangles covering 192x192 world units.
Vertices are a readonly record struct (position, normal, texcoord)
so the upcoming GPU upload in Task 8 can sizeof() them directly.
Height byte -> world z uses a simple 2x scale; the real AC height
lookup table is a Phase 2+ concern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Terrain/Vertex.cs

@@ -0,0 +1,5 @@
+using System.Numerics;
+
+namespace AcDream.Core.Terrain;
+
+public readonly record struct Vertex(Vector3 Position, Vector3 Normal, Vector2 TexCoord);