Tiny World Ghost World Generation

makeGhostWorld(boardX, boardZ) produces the contents of a single non-editable ghost board. It must be:

• Deterministic. Same (boardX, boardZ) always yields the same cells. Cached in ghostBoardCells keyed by 'bx,bz'. Panning away and back must regenerate identically (we rely on this for the sticky-reveal jigsaw — if content shifted between regens the reveal cache would lie).
• Connection-aware. Paths and rivers must line up across board edges. The user should be able to walk a road from one ghost board into the next.

The seeded RNG