feat: support forking of the dogd client

[sky-mart]

"

[github-actions]

• Blocking: DogstatsdClient::sender() takes self.worker via std::sync::Mutex before it can check the PID and respawn the worker. If a multithreaded parent forks while another thread is emitting and holds that mutex, the child inherits the mutex locked by a thread that no longer exists, so the first post-fork emit deadlocks at orb-dogd/src/dd.rs:180 before reaching the fork-detection logic. The new tests avoid concurrent dogd emission during fork, so they do not cover this failure mode. This needs a design that does not rely on a fork-inherited mutex for recovery, or the supported contract must explicitly exclude concurrent emits around fork and test that boundary.

[sky-mart]

useless, because we always use fork+exec