Add foundational voxel engine: generic, terrain, and smooth voxel support

[Copilot]

Implements core voxel engine systems supporting discrete blocky materials, density-based terrain, and smooth surfaces via Marching Cubes. Designed for extensibility (LOD, streaming, editing).

Core Data Structures

• VoxelData: Density (fp16), material ID, flags. 4 bytes per voxel.
• ChunkData: 3D voxel grid (default 32³), flat array with indexing helpers, mesh references, NativeArray support for Jobs.

Meshing Pipeline

• GreedyMeshingJob: Face culling for blocky voxels. Burst-compiled, zero GC.
• MarchingCubesJob: Simplified MC prototype for smooth terrain. Burst-compiled, zero GC. Full lookup tables noted for future implementation.
• IMeshingJob: Interface for pluggable meshing algorithms.

Chunk Management

• ChunkManager: Spatial hash, automatic job scheduling, world/local coordinate conversion, smart mesher selection (blocky vs smooth).

Example Usage

ChunkManager manager = gameObject.AddComponent<ChunkManager>();
manager.ChunkMaterial = material;

// Blocky voxels
manager.SetVoxel(new int3(x, y, z), VoxelData.CreateSolid(materialId: 1));

// Smooth terrain with density field
manager.SetVoxel(new int3(x, y, z), VoxelData.CreateSmooth(density: 0.7f, materialId: 2));

// Meshes generate automatically via Jobs

Implementation Notes

• Half precision density saves 50% memory (68KB vs 128KB per 32³ chunk)
• Sequential job processing for simplicity; parallelizable via NativeList
• MC uses basic surface approximation; full 256-entry tables commented for production use
• Assembly references added: Unity.Burst, Unity.Collections, Unity.Jobs, Unity.Mathematics

Includes 15 unit tests, 3 example demos, and architecture documentation (ARCHITECTURE.md, README.md).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Foundational voxel engine systems: Generic, Terrain, and Smoothed voxel support</issue_title>
<issue_description>## Is your feature request related to a problem? Please describe.
To develop a robust voxel engine that can support discrete blocky materials and natural, smooth terrain, foundational systems must be established. The engine should be extensible, high performance, and capable of both grid-based and density-based voxel representations.

Describe the solution you'd like

This feature should establish core systems for a voxel engine supporting:

• Generic voxels (blocky, discrete materials)
• Terrain voxels (density fields for natural landscapes)
• Smoothed voxels (continuous surfaces via Marching Cubes or Dual Contouring)

The initial implementation should provide:

• Data structures for different voxel types
• Chunk management for spatial partitioning
• Meshing pipeline hooks to support future LOD, streaming, and editing features

Tasks

• Define Voxel struct with density, material ID, and flags
• Implement Chunk class with chunk coordinates, voxel storage, and mesh references
• Add indexing helpers for 3D voxel arrays
• Create placeholder meshing jobs:
  • Greedy meshing for blocky voxels
  • Marching Cubes prototype for smooth terrain
• Integrate Unity Jobs + Burst for meshing execution
• Generate meshes per chunk and assign to MeshFilter/MeshCollider
• Document architecture decisions (chunk size, density representation, meshing choice)

Requirements & Notes

• Meshing must run off the main thread via Jobs + Burst.
• Code should be structured for extensibility (future LOD, streaming, editing).
• Keep chunk size configurable (default 32³).
• Use Half (fp16) for density to balance precision and memory.
• Mesh generation can be minimal (cubes + basic Marching Cubes), but must demonstrate both blocky and smooth paths.
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Foundational voxel engine systems: Generic, Terrain, and Smoothed voxel support #39

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