- Remove extra-edge generation and cross pieces; board is now a single random spanning tree (one unique route between every pair of cells, no loops)
- Update leak rendering to place droplets on exact open edges using DELTA/EDGE and render water/flash layers above tiles in the container
- Adjust mini preview tiles and difficulty config (drop `extra` param)
- Rework tutorial to explain tree structure, scattered dead ends, and branch-by-branch solving strategy
- Update verification tool: assert tree edge count, no 4-way cells, leaf distribution across rows, faucet row variety, and board randomness; replace cross-based fixture with a valid tree
