NavMap A*: terrain costs and limits, and links between surfaces

[fmrico]

Closes #129

All options are off by default, so existing configurations plan as before.

• Slope cost. terrain.slope_weight adds slope_weight × tan²(slope) per meter on each NavCel. Gentle slopes cost little and steep ones a lot, whether the robot goes up, down or across them. A gentle ramp is therefore preferred to a steep one that reaches the same height. With 100, 5° adds 0.8 per meter, 10° adds 3 and 20° adds 13; flat ground costs cost_factor (2).
• Shortcut. The line-of-sight shortcut never replaces part of the path by a segment that is steeper or costlier than the waypoints it removes. Before, it could straighten a detour back over the slope.
• Hard limits. terrain.max_slope_deg, max_step, max_roughness and min_clearance make the NavCels beyond the robot's limits untraversable. They are computed from the NavMap descriptors once per map geometry.
• Links. Links between surfaces of the allowed kinds are search edges (links.types, by default generic, ramp and door). The path keeps the two ends of a link, with no waypoints in the air and no shortcut across it.
• Graph cache. It now detects a new map by its geometry id.

Planning time on real maps is unchanged, about 1 ms on the excavation (30 000 NavCels) and on the parking garage (38 000). The first plan after a map loads takes 6–35 ms, depending on the limits enabled. On the excavation, with slope_weight 100, the steepest NavCel on a 20 m path drops from 21.6° to 9.1°, at the cost of a 3 m detour.

Nine new tests cover:

• the terrain is ignored by default;
• a hill is avoided;
• a gentle ramp is preferred to a steep one to the same height;
• the slope, step, roughness and clearance limits;
• links by kind and direction;
• reconfiguration;
• a new map.