received() acquired mutex_header and mutex_session in different orders
depending on packet type:
- pt_Data path: mutex_header → mutex_session
- pt_Start path: mutex_session → mutex_header
send_process() acquires mutex_session independently. When running
concurrently with received(), the reversed lock ordering creates a
classic ABBA deadlock scenario.
Fix: enforce consistent ordering (mutex_header → mutex_session) in
all code paths. Restructured pt_Start case and buildSession() to
always acquire mutex_header before mutex_session.