Add NavMap::distance_field

[fmrico]

Fixes #32

New representation primitive:

std::vector<float> NavMap::distance_field(
  const std::vector<NavCelId> & seed_cells, bool from_boundary, float max_distance) const;

It returns the 3D distance from each NavCel centroid to the nearest seed:

• the centroids of seed_cells;
• if from_boundary, the open edges of the mesh, as real segments (walls, drops, unmapped space).

How it works:

• A Dijkstra-like propagation carries the nearest seed's geometry over the adjacency and measures the distance straight to it, not as a sum of centroid hops.
• It never crosses between disconnected surfaces, so an obstacle on one floor does not affect the floor above.
• Cells beyond max_distance, or unreachable ones, get +inf.

The EasyNav InflationFilter will use it (separate PR in easynav_plugins). The likelihood-field localization on the NavMap and clearance-aware planning can reuse it.

Tests

test_distance_field (8 tests, all against brute-force distances):

• rectangle borders (exact);
• seed cells, and seeds together with the borders;
• the max_distance cut-off;
• stacked floors not seeing each other;
• a 30° slope, where the distance is 3D;
• interior hole edges;
• irregular terrain with holes: never below the true distance, mean excess < 2 cm;
• degenerate inputs.

They also pass under ASan/UBSan.