Free-threaded GC improvements

I’ve been spending a little time investigating potential improvements to the free-threaded GC, using Claude Code to prototype and measure the effects. None of the code is in a state that I would be eager to merge into CPython, but I do think it shows some potential improvements, and some trade-offs we might want to consider (primarily memory vs. runtime). I’ll continue work on this (I have some time on my hands[1]), and I’m sharing all this in case other people are interested as well. Benchmark...