feat: Add ShutdownWatch handle

[tisonkun]

Summary

This proposes a ShutdownWatch handle created from ShutdownRecv::watch().

ShutdownWatch observes the shutdown latch without holding the wait group, so cloning or storing it does not delay ShutdownSend::await_shutdown.

API shape

impl ShutdownRecv {
    pub fn watch(&self) -> ShutdownWatch;
}

#[derive(Debug, Clone)]
pub struct ShutdownWatch;

ShutdownWatch mirrors the observation APIs on ShutdownRecv:

• is_shutdown_now
• is_shutdown
• is_shutdown_owned

The naming is intended to stay closer to the existing short role names (ShutdownSend / ShutdownRecv) than Observer, while avoiding the misleading Rust Weak association.

Closes #112.

Verification

• cargo x test
• cargo +nightly x lint