Validation: integrate external monthly data and overwrite HIST where available

Siddik, ABrequested to merge
val/misc-gen-integration into main

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

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