fix: restore alpha_wrt scalar registration missing from refac-params

The refac-params branch dropped the bare alpha_wrt scalar param from
definitions.py — it was in master's _wrt loop but not carried over.

This caused ./mfc.sh validate to reject example cases using 'alpha_wrt': 'T'
(the schema had patternProperties for alpha_wrt(N) but not the scalar form),
matching what master registers: both the bare scalar and the indexed forms.

Author: sbryngelson

toolchain/mfc/params/definitions.py

@@ -623,6 +623,7 @@ def _load():
         _r(n, LOG, {"output"})
     for n in [
         "schlieren_wrt",
+        "alpha_wrt",
         "rho_wrt",
         "E_wrt",
         "pres_wrt",