Add find_freeroute_cli() + _run_freeroute_cli(): run_freerouting now tries the
Java-free freeroute autorouter first (freeroute's CLI is -de/-do compatible)
and falls back to the JAR for boards freeroute cannot yet route. freeroute is
invoked as a subprocess, not imported, keeping its GPL license cleanly separated
from mckicad (MIT). Raises a clear error only when neither backend is available.
End-to-end verified: DSN -> freeroute -> SES -> native applier = 141 segments on
the Arduino_Mega board, fully headless and Java-free.
