Extends a small three.js scene into a long, streamed world (hundreds of metres along a river, road or valley) — one signed-distance function for water/land, a curvilinear chunked terrain strip with seamless normals, a camera-following snapped water grid, a shadow frustum that follows the view, landmark pads, a pooled set of crafted GLB trees re-seated near the camera, deterministic seeded scatter and distance culling. Use when the user asks to "extend the world", "make the river longer", add a lake/spring/village/terraces, stream terrain in chunks, place landmarks on terrain, or when far reaches look empty, flat-shadowed or cost too much.
