[M3-S2b] Hidden Garden: parallax layers from a depth key #142

Problem

The Hidden Garden chunk is one flat rear layer above the path, all at scroll 1.0. Its own README notes: "sky/far/mid parallax waits for hand-drawn masks." The approved concept and docs/04's layer table give it depth: sky 0.1, far 0.25, mid 0.6, foreground 1.2.

Acceptance criteria

• scripts/depth_layers.py: the model repaints the chunk as a flat-colour depth key; masks come from that key, edge-refined against the real pixels (never hand-painted); the model fills what nearer layers hide
• layers.json uses the masks; split_layers.py --check still rebuilds the art exactly; export_stage.py --verify passes at rest
• docs/09 and the region README: a mermaid diagram of the steps, the palette and the scroll values
• A comparison: the composed chunk against the approved concept, plus a parallax preview at three camera positions

Notes

Lane A. Multi-chunk parallax needs lane B's placement fix (commented on #116).