Rewrite from_points as a multi-level grid mesher

[fmrico]

Fixes #30

from_points (and from_pointcloud2) grew triangles from seed fans and border edges and produced overlapping, non-manifold meshes. This replaces it with a deterministic multi-level grid triangulation that is manifold by construction.

How it works

1. Levels. Points are binned in XY cells of resolution. In each cell, points closer than max(min_clearance, max_dz) in z form one level: stacked floors give one vertex per floor.

2. Height of a level. A robust plane fit (two passes, trimming outliers) of its upper face, at the cell centre.

   • A slab seen from above and below keeps its upper face.
   • A level that is not a plane (wall, furniture, bush) keeps a high top, so it does not join the floor.

3. Sparse faces. Two neighbour-consensus passes:

   • a sparse upper face is adopted when a neighbour stands on it;
   • a cell that only saw the lower face of a slab takes its neighbours' upper face, but only if there is a level below it in its column.

   Spikes, potholes and boxes are never lifted.

4. Holes. Enclosed empty regions up to max_hole_area (1 m² by default) are filled from the levels around most of their border.

5. Links and triangles.

   • Each level links to the closest level of each of the 8 neighbouring cells, within the step and slope limits, only if the choice is mutual.
   • Each quad gets at most the two triangles of one diagonal per level: no overlap, at most two triangles per edge.
   • No triangle is steeper than max_slope_deg.

6. Output.

   • Only used vertices are kept.
   • Surfaces are the connected components; max_surfaces and the new min_surface_area drop the triangles and vertices of the removed surfaces.
   • Layers: elevation, plus the new support (points behind each triangle, 0 if interpolated).

New BuildParams: min_clearance, min_surface_area, max_hole_area. max_edge_len, neighbor_radius, k_neighbors, use_radius and min_angle_deg are kept for API compatibility and are now ignored. Voxelized clouds need a resolution of at least twice the voxel size.

build_adjacency now sorts the edge uses instead of hashing, resets the neighbors before rebuilding, leaves non-manifold edges open (no asymmetric links) and counts them (nonmanifold_edges()).

The .navmap file format and the message are unchanged.

Results

Max triangles per edge in brackets.

Cloud	Before	After
Flat 20x20 m at 1 m	2193 triangles, 1592 m² (6)	800 triangles, 400 m² (2)
Excavation, 0.3 m	75 s, 7.2 M triangles, 4.5 M m² (46)	0.13 s, 113 k triangles (2)
House, 0.3 m	1.3 s, 166 000 m² for a ~300 m² house (47)	14 ms (2)
Simulated VLP-16 over the excavation, 5 cm noise + 1% spikes, 0.5 m	height error p50/p90 2/11 cm	1/3 cm, +18% coverage
12.7 M points	—	1.7–2.2 s, 0.6–0.7 GB

Tests

• test_from_points (30 tests):
  • flat, noisy, irregular and voxelized clouds;
  • stacked floors and a ramp joining them;
  • steps, slope limit, wall, box, table vs min_clearance;
  • slab faces, sparse upper faces, pothole;
  • hole filling and its limits, multi-floor holes;
  • outliers, spikes, non-finite points, point order;
  • max_surfaces / min_surface_area, support layer, degenerate inputs.
• test_adjacency (6 tests): non-manifold edges, stale links after a rebuild, copy.
• navmap_core and navmap_ros pass under ASan/UBSan.
• The tests of navmap_tools, navmap_examples and the EasyNav NavMap plugins pass unchanged.