Numeric country code from the Gleditsch-Ward (G&W) state system, where it diverges from COW. Most countries share codes; this field captures divergences. NULL means G&W uses the same code as COW. Note: the Thyne do-file encodes most G&W divergences as year-conditional ccode remaps rather than country-name remaps; those time-varying rules are documented in EDGE_CASES.md.