Terrain: DDA voxel raycast + voxel editing brushes with undo/redo

[drsnuggles8]

Is your feature request related to a problem? Please describe.

There is no voxel-grid raycast. Renderer/Ray.h plus BoundingVolumeHierarchy handle mesh picking, but a voxel volume needs grid traversal, not BVH descent — and without it there is no precise voxel picking, so Terrain/Voxel/VoxelOverride edits cannot be driven interactively from the viewport.

Describe the solution you'd like

• Amanatides & Woo DDA traversal — step the ray cell-by-cell in the voxel grid, crossing chunk boundaries correctly, returning the hit voxel and the face normal (needed to know which side to place against).
• Editing brushes on top of it — place / carve / fill / paint with a radius, marking touched chunks dirty for incremental remesh rather than rebuilding the volume.
• Undo/redo over voxel edits, integrated with the editor's existing command stack.

Describe alternatives you've considered

Raycasting against the generated mesh via the BVH — works for picking a surface point but not for identifying the voxel behind it or the placement face, and the BVH is stale the moment the volume is edited.

Additional context

Reference: D:\repos\VoxelEngine\VoxelEngine\voxel_ray_cast.h (DDA traversal) and voxel_edit.h (brushes + dirty tracking + undo), with the chunk-dirty/remesh loop in chunk_manager.h. MIT.

Pairs with the greedy-meshing issue (incremental remesh is much cheaper when the mesher is fast) and with Terrain/Voxel/VoxelOverride.

Acceptance criteria

• Clicking in the viewport identifies the exact voxel and the hit face across chunk boundaries.
• Edits remesh only the affected chunks.
• Undo/redo round-trips a multi-chunk brush stroke exactly.

Score

capability: 3
craft: 3
stability: 1
decay: 1
effort: 3
confidence: 0.8
learning: 3
fun: 5
kano: table-stakes
blocked_by: []
blocks: []

_{Rated per issue-scoring · score = confidence × (capability + craft + stability + decay) / effort, derived by the picker.}

🤖 Generated with Claude Code

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🔗 Related PRs

#313 - feat(renderer): add CPU mesh BVH for ray-vs-mesh queries [merged]
#329 - Scatter brush: mesh-surface raycast via the triangle BVH (closes GPU-… [merged]
#660 - feat(renderer): realtime DDGI probe volumes (#632) [merged]
#669 - feat(physics): per-tile streamed terrain collision + sculpt heightfield updates (#469) [merged]

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