I'm just going to vent my relief on the thread, having held it in and acting cheery all day.
So, sometime in the middle of last night I heard a crashing/tumbling noise, followed by a pause, followed by another crashing/tumbling noise. This, I thought, was part of a dream.
This morning I noticed Phoebe didn't come out from her current favourite spot pressed against the curved radiator in my bedroom. She did make her usual Hello! chirrup, so I gave her a quick chin-tickle and topped up her food and water, before we went out (DC have an activity first thing on Saturdays).
On the back from the activity, while DC were talking about their dreams, I mentioned the crashing dream and they reminded me that we in fact have a Ring camera covering our front hall (we never have notifications on that camera unless we're away on holiday, so I tend to forget it's there).
Well, to my horror, it turned out I hadn't dreamt the noise - it was Phoebe, she had fallen first down the short bit of the stairs to the landing, then tried to go down the longer bit of stairs and tumbled like a sack of spuds all the way down to the bottom. When we got home, we rushed to check on her and realised that she couldn't walk properly: her back half was basically swaying about like a drunken sailor. I was convinced she'd suffered a spinal injury.
I managed to get an emergency appointment at the vet this afternoon. So after a lot of very detailed prodding and squeezing, the vet came to the reassuring conclusion that nothing is broken or torn or internally injured, all pumps and plumbing are in working order, and Phoebe is probably just a little sore and bruised. So that's a massive relief.
On the downside, the vet also concluded (having reviewed the Ring footage) that Phoebe isn't wobbly because she fell down the stairs. She fell down the stairs because she's wobbly. In other words, it's probably a sudden brain thing, like a small seizure or stroke.