Creating a fruit saison: when to add the fruit and rack to secondary?

There is really no right or wrong answer here, especially with the anecdotal (and some scientific) evidence coming to light over the past years. Most books on homebrewing, and indeed most advice online, on adding fruit repeats the advice given for dry hopping: Wait until active fermentation has stopped (no bubbling out of the airlock, gravity is stable over a couple of days) in order to minimize the amount of flavor compounds lost through CO2. This advice mainly aims at introducing "pure" fruit aroma which might fade pretty quickly with time. Recently more and more people have started adding fruit either at the time of pitching the yeast or some other time during active fermentation in order to get an interaction between yeast and fruit aroma compounds. The same trend can be seen in dry hopping where this is usually referred to as biotransformation. I have definitely had good experiences with either process but I suggest that you try out what works best for you on a small scale. Especially with something like a puree which can easily be split, I would recommend using small fermentation vessel (resealable food containers, or similar) to try out different times to add fruit and different exposure times.

In general you should wait for fermentation to be completed in the primary, then rack to your secondary. So it means waiting to have a stable specific gravity for 3 days.

In my opinion, I don't think you need to wait any longer to allow yeast to clean the byproducts of fermentation, as it can be done in the secondary as well. Yeast will do it's job, no matter if it is a primary or a secondary...

It has also been discussed here : If/When to move to secondary fermentation.

If you're racking onto fruit then a second fermentation from the fruit is expected, so the main thing you need to wait for is a noticeable active fermentation to end. The main reason for this is that heavy fermentation will pull off the fruit aromas you're trying to keep as the CO2 leaves solution. For a fast fermenting yeast you might need to wait 3 days, for a slower one maybe 2+ wweks. If you're looking for the simplest answer, then I'd say as soon as the krausen has subsided you're good to go.

Having said that; racking to secondary is not necessarily required. Pulling your brew off the trub is more of an issue with breweries, as yeast autolysis occurs more quickly when there is higher volumes of beer. On a homebrewing level, you should not start to see significant autolysis for multiple months.