Evaluate whether to use `bun` ≥ 1.4

[lgarron]

bun was vibe-ported to Rust for version 1.4. This makes me a bit antsy.

Then again, bun's own has always been fly-by-the-seat-of-their-pants, and somehow this port has resulted in more issues being vibe-fixed than before.

I don't think it's viable to pin to bun <1.4, so we need to decide whether to go all-in on ≥1.4 and when. I suspect it's better to rip-off the bandage sooner rather than later (assuming nothing breaks), and if it turns out not to be stable we can figure out our options later.

[lgarron]

I suspect it's better to rip-off the bandage sooner rather than later (assuming nothing breaks), and if it turns out not to be stable we can figure out our options later.

Turns out it would be a bunch of work to prevent bun 1.4 from running in CI. I might do that if it starts happening in more projects, but so far I've only seen one regression that doesn't have an alternative explanation.