recursivelyCopyDirectory

Recursively copy a directory tree with the same collision-resolution rules the engine uses (pack.mcmeta merge, JSON deep-merge for mergeable files, higher-priority wins for everything else). Used by ThirdPartyResourcePack.processCluster to combine multiple cluster sub-packs into a single temp directory before zipping it as one pack input for the engine. Kept public for the same callsite reason — moving it into resourcepackmanager-mixer would force the Bukkit-side cluster code to import the mixer's internal helpers.