Pilot: the core rules, a solver and 12 proven levels
A playable throwaway prototype of the core rules, with placeholder shapes: tap a lane to send its front imp, which digs the highest exposed block of its color. An imp with nothing to dig waits in one of 5 slots and digs by itself once its color is exposed. Dig out the chest to win. Fill every slot with imps that can't dig and you're stuck. Every color also has its own symbol.
A solver that plays every possible line of a level through the game's own rules module, saves a winning line as proof, and measures the level: how often random play wins, whether a "dig anything you can" player wins, trap depth, choke points, slots needed and length.
12 levels, from three hand-made tutorials to an Expert finale whose winning line needs all 5 slots at once. A check re-solves all 12 in under a second and fails if any level stops being winnable or its numbers drift from the rules.
The GIF shows level 3: two imps wait, a yellow dig sets off a chain that frees them, and the last blue imp digs out the chest.
TREASURE!