Add watch channel with latest-value semantics

[veeceey]

Adds a watch channel type that stores only the latest value. Sending never blocks (overwrites the previous value), and receiving blocks until a new value is available.

This is useful for scenarios like refresh signals where you only care about the most recent value - if multiple sends happen while the receiver is busy, the receiver processes just one iteration instead of N.

The implementation uses Mutex<Option<T>> + Condvar as suggested in #942. Key points:

• WatchSender and WatchReceiver are both Clone
• Each receiver independently tracks which version it has seen
• Disconnection is properly detected (recv returns RecvError when all senders are dropped, send returns SendError when all receivers are dropped)
• try_recv is available for non-blocking checks

let (tx, mut rx) = crossbeam_channel::watch::<i32>();
tx.send(1);
tx.send(2);
assert_eq!(rx.recv(), Ok(2)); // only latest value

Closes #942

[veeceey]

Test results:

running 12 tests
test watch_clone_receiver ... ok
test watch_clone_sender ... ok
test watch_multiple_sends_between_recvs ... ok
test watch_overwrites_value ... ok
test watch_recv_error_on_disconnect ... ok
test watch_send_error_when_no_receivers ... ok
test watch_send_recv_basic ... ok
test watch_multithreaded ... ok
test watch_try_recv_disconnected ... ok
test watch_try_recv_empty ... ok
test watch_try_recv_value ... ok
test watch_recv_blocks_until_send ... ok

test result: ok. 12 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

Full existing test suite also passes (74 doc+unit tests).

[veeceey]

checking in on this - the watch channel is a pretty commonly requested primitive. open to design feedback

[veeceey]

friendly bump on this - happy to make any design changes if needed