Validation: integrate external monthly data and overwrite HIST where available
Adds support for external monthly generation sources (e.g., post-2008 data for 279_SALT SPRINGS) that are missing from EIA-923. New data are ingested via data/misc/miscellaneous_generation.csv and used to overwrite RectifHydPlus_actual_backfilled for matching (eia_ptid, year) rows.
Closes #2