fix: LiveStream tail loop ran forever with no way to stop it

[alubbock]

Summary

• LiveStream.__init__ ran the file-tailing loop inline, blocking forever. Because the constructor never returned, stop() was unreachable from outside — making it useless.
• Redesigned to run the loop in a background daemon thread so __init__ returns immediately with a usable object
• Added stop() to signal termination and join() to wait for the thread to finish
• Replaced time.sleep() with threading.Event.wait() so stop() responds promptly rather than waiting out a full sleep interval
• Exceptions in the processing loop are now caught and logged rather than silently killing the thread
• Adds microbench/tests/test_livestream.py with two tests covering normal processing and stop() termination