Phase 3: add a column/span LOD mesh builder

[MichaelFisher1997]

Goal

Add a new mesh-building path that consumes richer LOD columns/spans while keeping the current heightfield mesh as a fallback.

Work Items

• Implement a column/span mesh builder next to the existing heightfield builder.
• Generate top, side, water, and optional internal faces from representative spans.
• Add greedy or local quad merging where it is simple and safe.
• Use adjacency data to avoid unnecessary edge faces and reduce cracks.
• Add tests for steep terrain, water, empty columns, span ordering, and fallback behavior.

Acceptance Criteria

• The new mesh builder can be enabled per LOD level or via config/test hook.
• Mesh output avoids obvious holes for multi-layer columns.
• Heightfield mesh remains the stable fallback path.
• zig build test passes.

[github-actions]

Analysis Summary:

This is a Feature Request for Phase 3 of LOD column/span mesh building. The current heightfield-based buildFromSimplifiedData() in lod_mesh.zig collapses multi-layer column data into single heights, losing information about subsurface/foundation layers and internal faces.

Existing Architecture (modules/world-lod/src/lod_mesh.zig):

• LODMesh.buildFromSimplifiedData() - Heightfield mesh with smoothed quads (lines 192-263)
• LODMesh.buildFromSimplifiedDataWithQEM() - QEM simplification wrapper (lines 268-331)
• LODSimplifiedData (world-core/lod_data.zig) already contains material_layers[] with surface/subsurface/foundation and water[] with surface_height/depth

Implementation Suggestion:
A new LODMesh.buildFromColumnSpans() function that:

1. Iterates columns, extracting vertical spans from material_layers[i].{surface, subsurface, foundation}
2. Generates vertical side faces between adjacent columns with height/material differences
3. Uses greedy quad merging for contiguous same-material vertical faces
4. Handles water as separate span at water[i].surface_height
5. Falls back to buildFromSimplifiedData when data is unsuitable

Scope: Well-scoped with specific acceptance criteria. The issue appropriately requires heightfield as fallback. The LODSimplifiedData structure already has all necessary data - only the mesh generation logic needs to be extended.

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