kimjunte + Khalim alignment: when the band column is mapped and a description
can't be resolved to A-G, write the row with an explicit Unknown value rather
than silently skipping. The landlord declared the column, so every row gets a
value; 'assessed, no band' is now distinct from 'never assessed'. Still never
fails the finalise (unlike the mandatory components). Modelling reads Unknown as
no band (no efficiency anchor), so it stays inert to SAP.
Requires the FE boiler_efficiency_band pgEnum to include 'Unknown' (assessment-
model#486 updated). 7 finaliser tests green; pyright clean.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>