Make fan curve service robust to hwmon renumbering

Update `/usr/local/bin/bc250-fancurve` so it discovers the correct hwmon paths at startup rather than relying on fixed `hwmonN` numbers.

Requirements:
- Find the fan controller by matching `/sys/class/hwmon/hwmon*/name` to `nct6686`.
- Find the CPU temperature sensor by matching the hwmon name to `k10temp`.
- Resolve `pwm2`, `pwm2_enable`, and `temp1_input` from those discovered directories.
- Fail with a clear error if either device is unavailable.
- Run under systemd and restart after unexpected failure.

Keep durable system details in `PROJECT.md`; this issue should only track the actionable change.

Start with /usr/local/bin/bc250-fancurve and inspect how it currently selects hwmon paths. Check the relevant systemd service configuration, then verify startup behavior when nct6686 or k10temp is unavailable and confirm the service restarts after an unexpected failure. Done means both devices are discovered by name, the listed paths resolve correctly, and failures produce a clear error.

Written by the indexing model from the issue text.