Parallel execution layer

aether/parallel/executor.py (v3.9.0) gives skills true threading-based concurrency — drive while polling a sensor. Its safety model:

• Daemon workers — killed at interpreter exit without running their finally blocks, so the authoritative motor kill is the adapter emergency_stop from the app's exit handler.
• _GLOBAL_STOP — every worker checks a process-wide stop, so Ctrl-C halts all daemons even if run() was unwound.
• _MOTOR_LOCK — serializes motor commands so a drive and a stop never interleave at the I2C layer.
• Hard timeout — a wall-clock cap; workers are joined on exit.