[M4-S0] Rusty Creek: region-wide plates for its parallax layers #150

Problem

Rusty Creek draws 01-distance (scroll 0.2), 06-near-rails (1.1) and 07-foliage (1.15) per chunk. Placed at its chunk's origin, a layer at scroll < 1 overlaps the next chunk's copy by 2160 × (1 − scroll), and one at > 1 leaves a gap of 2160 × (scroll − 1). In the game the sky jumps at hard vertical edges and the foreground has holes.

Acceptance criteria

• Each of those layers is one region-wide plate, quilted with depth_layers.py region from the chunks' own plates (aliases resolved) and cut into 2160 px tiles, tile j carried by chunk j
• export_stage.py --verify still rebuilds every chunk at rest; compare.py parallax shows no edges or holes along the whole creek
• docs/09 mentions it

Lane A; follows the method in #148.