An oncologist friend of mine jokingly says that he cured cancer if his patient dies from being hit by a bus before the cancer gets him.

A bit of that logic applies here. The fact that the crack propagated to the point that the seat tube extension was weakened over 80% then stabilized, points to it's no longer being an issue. If you consider that the seatpost carries the load past the crack you can see why it's no longer spreading.

I suspect that the seatpost itself is slightly more flexible than the seat tube, and when flexed back overloaded the tube and cracked it above the weld. Now that it's cracked the upper part of the tube is free to flex with the seatpost and that's why the crack isn't spreading.

As long as the seatpost is of decent quality, and adequately strong for the OPs load, and is insereted so the minimum insertion is below the crack as it seems to be, he should be OK.

OK, that is, as defined by my friend the doctor. Odds are that this crack will not be the cause of a sudden failure, and I'll bet that when failure comes it'll be elsewhere in the frame.

One step I would take is to lift and inspect the seatpost from time to time.