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.
New standalone debug prototype exploring the "gem-cut" flying island
shape: a flat hexagonal table (center region) surrounded by 6 sloped
trapezoidal facets down to the girdle (6 outer regions), with a
non-walkable 6-sided pavilion pyramid below for the underside. Crown
gets trimesh collision for later player testing; all 6 outer facets
stay visually neutral (alternating flat color only) since theming
comes later. Doesn't touch main.tscn or the old cross-layout prototype.
