Sliding puzzle solvability criterion

Also called Parity criterion for the 15 puzzle.

A configuration is solvable exactly when the permutation parity matches the blank's row parity, for the odd-width case.

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The build computes these levels from the evidence in the repository. Each row says what the next level needs. Read what the levels mean.

Ladder	Level	What the next level needs
Human proof	H0 no proof	add a proof in proofs/ (06.5)
Lean	L0 no Lean part	name a Lean module in the lean field (07)
Visual	V0 none	add viz/scene.ts or a figure (10)
Writing	W0 stub	Statement one source with a locator