Build one SigDb bundle. tier: 'current' keeps only each package's latest version, 'full' every
version; topN + shard further restrict to the top-downloaded packages or the remainder, for a SigDbManifest.
Build several shards that all reindex into a single shared string dictionary (stored once, not per shard, pooled + frequency-sorted together); the compact, fast-loading counterpart of calling build per shard.
Accumulates analyzed functions and serializes a SigDb. Feed it with addPackage/addVersion, then build; pooling happens there so the result is deterministic for identical inputs.