Promote debug/HexDiamondIsland.gd to terrain.gd at project root now
that main.tscn depends on it, and swap it in for the old flat
checkerboard Ground (Player/Camera/POI untouched, table sits at the
same y=0 the old ground surface used).
Add _build_region_borders(): places mountain.glb/mountain2v5.glb/
forest.glb (same models, scale factors and hitbox sizes already used
elsewhere in main.tscn) continuously along the 6 ridges between table
corners and girdle corners, i.e. the shared edges between adjacent
outer regions.
Godot 4.6 exploration game with player movement, step-rotation camera
rig, and POI interaction system (explore/rest/close menu). main.tscn
is a test scene with placeholder props; debug/ holds unwired terrain
prototypes (voxel river, procedural island mesh, cross-shaped hub
