gitshallow.Repo.GCInterval int:
0 (default) = git auto gc (no explicit call)
N = aggressive gc + prune every Nth successful pull
GC() simplified to always aggressive+prune (the only mode we use).
Sync(), Init(), Fetch() all parameter-free; GCInterval baked into Repo.
gitshallow: fix double-fetch (pull already fetches), drop redundant -C flags
gitdataset: split into GitDataset[T] (file+atomic) and GitRepo (git+multi-dataset)
- NewDataset for file-only use, AddDataset to register with a GitRepo
- one clone/fetch per repo regardless of how many datasets it has
ipcohort: split Cohort into hosts (sorted /32, binary search) + nets (CIDRs, linear)
- fixes false negatives when broad CIDRs (e.g. /8) precede specific entries
- fixes Parse() sort-before-copy order bug
- ReadAll always sorts; unsorted param removed (was dead code)
