Remove HDPackManager, expansion overlay manifests, and BLP size-comparison
logic. Assets now resolve through a single manifest with a simple override
directory (Data/override/) for future HD upgrades.
Extracts each expansion's assets as a CRC-compared overlay against a
base manifest, storing only files that differ. Auto-detects overlay mode
when a base manifest already exists. Adds --as-overlay, --full-base
flags and manifest merge for partial extractions.
Expansion overlays allow each expansion to supplement the base asset data
via an assetManifest field in expansion.json, loaded at priority 50 (below
HD packs). The asset extractor gains --reference-manifest for delta-only
extraction. Also fixes CharSections field indices (VariationIndex=4,
ColorIndex=5, Texture1=6) across all DBC layout references.