I only have a moment, but I wanted to quickly relate what I've learned and figured out about my concerns over Sammy's effort.
First, I think I'm overreacting to just about everything due to the pregnancy hormones. Second, I know that I am hypersensitive to this issue to begin with because of my own childhood, and my development of an anti-effort mentality. So, after a few days of reflection, the whole issue is much smaller.
Even so, the incident at school is worth thinking about. I spoke briefly to Sam's teacher after she returned, and she assured me that Sam does not do this "all the time." That part of what Miss R. told me was just a miscommunication, and that was the part that had me concerned. Sam does wander, observe, space out, and all of those things, but she doesn't do it excessively. And, just as I suspected, this school does keep a close eye on this behavior to assess whether there is a problem or whether it is just what the child needs to do at the time. Still, as Sam's teacher has noted to me since she started school, Sam's tendency is towards needing a push here and there, whereas some children just leap into the work. That is fine and normal - that is just her temperament. And, as Sam's teacher has been telling me, Sam has blossomed this year and is making independent choices much more often. So I'm 100% happy with how her school is handling this. (But I still want a better way to observe her in the classroom.)
So does Sam have any kind of "problem" in the area of effort, persistence, or independence? I've returned to my belief, prior to last Friday's incident, that the fact that I'm seeing improvement in her levels of effort is the most important thing. I've also received some very helpful comments that have made me think about whether I am conflating a lack of persistence or effort with caution, introversion, or other, less problematic character traits. I think I do mix them all together, so that I see the effort issue as larger than it really is.
Also, from the comments, I have some new ideas about how to introduce new activities to Sam, about praise, about motivation, and some other things I can't remember now. I guess I need to compile all of those ideas into my own list because there were obviously too many for me to remember them all. So thank you all for the great comments and advice!