[M0-S6] Split any composed chunk into the layer stack #64

Problem

Only Rusty Creek has layers (creek_layers.py). The Hidden Garden prologue needs the same split.

Acceptance criteria

• One script splits a composed chunk into the docs/04 layer stack (sky, far, mid, rear structure, water, play, near props, foreground)
• Recomposition is verified pixel-exact, like creek_layers.py
• Clean and anomaly states share every layer that doesn't change

Notes and dependencies

Rules: 09 – Art pipeline. Needed by M3 (Hidden Garden chunks). Suggested owner: Alexander (pipeline, art, scenes, deploy).

Verification

Run on a Rusty Creek chunk: same result as creek_layers.py; tests pass.