Is there a word for permanent solution?

A term that implies a permanent solution is:

A resolution

Yes, solution. Anything else is a stop-gap, or a patch, or a temporary solution. The word solution only needs to be qualified to specify when it is not a permanent one. If it is, you just call it a solution.

Take, for example, this definition from Merriam Webster (emphasis mine):

so·lu·tion

: something that is used or done to deal with and end a problem : something that solves a problem

: the act of solving something

: a correct answer to a problem, puzzle, etc.

There is nothing that implies that a solution is not permanent. Quite the contrary, a solution is always permanent unless otherwise specified.

A permanent solution is said to be definitive:

serving to supply a final answer, solution, or evaluation and to end an unsettled unresolved condition