additional constraint of numberlink/flow free

[farteryhr]

afaik, in most of implementations found in the app stores, the puzzles follow the convention:

if two adjacent grid belongs to the same path, they must be directly connected. in other word, if the grids passed by distinct paths are colored accordingly, no 2x2 block of the same color can present.

this adds difficulty on puzzle creators, but more puzzles will be unique solution this way. for example:

1 | ┌ | ─ | ─ | ─ | ┐
│ | │ | ┌ | ─ | ┐ | │
│ | │ | │ | 2 | │ | │
│ | │ | 1 | │ | │ | │
│ | └ | ─ | ┘ | │ | │
└ | ─ | ─ | ─ | ┘ | 2

for reference, other works: https://semiexp.net/games/infinite_numberlink/index.html https://github.com/thomasahle/numberlink