Finally got my navgrid system to work!
The game works out where the navigable parts of the level are immediately after instantiating the level components, and provides a subsystem for NPCs to request a point they can potentially get to, and be given back a list of "move forward/turn left/turn right" instructions they should follow to get there.
Worked great when I get 3D point subtraction ordering and cross-product direction interpretation right... 🤦
Now to add some inodes as a test!