[arudaev]

What and why

scripts/split_layers.py cuts a composed 2160×720 stage chunk into the docs/04 layer stack (z0 sky … z7 foreground) and proves the cut is exact. The Hidden Garden (#65) needs this; until now only Rusty Creek had layers, made by its own creek_layers.py.

• Rules. A region's layers.json lists its chunks, their composed art per state, and ordered mask rules: rectangles, polygons, a colour within an area, or another image's alpha. A rule can apply to one state only.
• Exactness. Every pixel goes to exactly one layer: the last rule that claims it, else the chunk's default. Because layers never overlap, stacking them rebuilds the art pixel for pixel. The script asserts this before writing.
• Shared layers. A layer identical in clean and anomaly is written once (<chunk>.<layer>.png); one the anomaly changes gets a file per state.
• CI. --check (no arguments = every region with a layers.json) proves the committed layers still rebuild the art, and now runs in the art workflow.

Issue

Closes #64

Stack

• [4/4] feat: export stage regions with the docs/13 manifest #110 [4/4] feat: export stage regions with the docs/13 manifest
• [3/4] feat: add the Hidden Garden collision and Dexter's waypoints #106 [3/4] feat: add the Hidden Garden collision and Dexter's waypoints
• [2/4] art: draw the Hidden Garden prologue chunk and its layers #105 [2/4] art: draw the Hidden Garden prologue chunk and its layers
• [1/4] feat: split composed stage chunks into the layer stack ← this PR

Merge bottom up with merge commits; retarget the next PR to main before merging it.

Same result as creek_layers.py?

The test splits the composed Rusty Creek left chunk, using the creek's own plate alphas as rules in its draw order. Every pixel lands in the layer creek_layers.py drew on top there, and the stack rebuilds after-left.png exactly.

The colours can't match the creek plates, and no splitter of a finished image could match them. Those plates are soft: the bank, for example, has 27 fully opaque pixels and 313,892 partial ones, so the composed art is a blend. Rusty Creek keeps creek_layers.py; new regions use the split.

Verification

• python -m unittest discover -s scripts -p "test_*.py": 6 new tests in test_split_layers.py:
  • exact recomposition with shared layers;
  • layers never overlap;
  • --check catches a stale layer;
  • state-only and colour rules;
  • bad config;
  • Rusty Creek.
• python scripts/split_layers.py --check and python scripts/check_docs.py