Log internal rotor frequencies with index and units

The previous debug line logged a bare float, which is hard to read
when multiple rotors exist and gives no indication of units. Now each
line includes the rotor index and cm^-1 units.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rwest

arkane/statmech.py

@@ -1210,7 +1210,7 @@ def project_rotors(conformer, hessian, rotors, linear, is_ts, get_projected_out_
 
     logging.debug('Frequencies from internal rotors:')
     for i in range(n_rotors):
-        logging.debug(int_rotor_freqs[i])
+        logging.debug('  rotor %d: %.6f cm^-1', i, int_rotor_freqs[i])
 
     if get_projected_out_freqs:
         return int_rotor_freqs