fix(core): make uv_compat ready queues thread-safe (#35175)
The per-handle waker ready queues in the libuv compat layer used
RefCell/Cell behind an unsafe Send/Sync assertion that the event loop
is single-threaded. On Windows that invariant does not hold: TTY
readiness is delivered by a thread pool wait callback registered with
RegisterWaitForSingleObject, and line-mode console reads complete on a
dedicated reader thread; both invoke the per-handle waker off the loop
thread. A wake racing the run_io drain panicked with "RefCell already
borrowed" at uv_compat.rs:609, and was a genuine data race since
RefCell's borrow flag is not atomic.
This switches the ready queues to Mutex<VecDeque>, the per-handle
ptr/in_queue fields to atomics, and drops the unsafe Send/Sync impls
so thread safety is enforced by the compiler. The mutexes are
uncontended on the single-threaded hot path. A regression test hammers
a waker from multiple threads while draining, mirroring the run_io
loop.
Fixes #35171
