morgul pedestal: check calib.log is a file before running

Setting as a directory gave

```
IsADirectoryError: [Errno 21] Is a directory:
'/Volumes/Junk/Data/i24-jf-202410/scr'
```

At the _end_ of processing

Start by tracing the processing path that reads calib.log and reproduce the case where calib.log is configured as a directory. Done means the invalid setting is detected before end-of-processing and reports a clear validation error instead of raising IsADirectoryError.

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