The STT/TTS worker _ensure() treated a spawned-but-not-yet-handshaked
subprocess as ready, so a voice turn arriving during warmup read the same
stdout StreamReader concurrently with the warmup handshake and crashed with
"readuntil() called while another coroutine is already waiting for incoming
data". Add a _start_lock + _ready flag so (re)start and the ready handshake
run atomically and callers wait for real readiness before reading stdout.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
6e20f2cd79