refactor: use libuv for subprocesses

[eric-wieser]

This PR changes IO.Process.spawn to use libuv, which fixes a fork-safety issue where setenv was called after fork.

This has the bonus of eliminating lots of windows-only code

Written with assistance from Antigravity / Gemini.

I have not yet carefully reviewed object lifetimes.

[eric-wieser]

release-ci

[eric-wieser]

@hargoniX, are you happy to send this to the release-ci? I think me adding the label doesn't actually work.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase a9946fe4ac0d53a63f81ccc6e8786ce513f8b494 --onto 0a6c31520b2ebaa6e71227454b46ebfb4986ae7b. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-01 07:35:07)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase a9946fe4ac0d53a63f81ccc6e8786ce513f8b494 --onto 3fc99eef102549c743c6c63547983319bfae6f01. You can force reference manual CI using the force-manual-ci label. (2026-05-01 07:35:09)