Source ledger

The durable per-source record of how each source has earned its standing: a log of SourceEvents (correction · corroboration) joined to the source that supplied each prior. Every StreamPrior already carries provenance (source + timestamp); the ledger is the join that makes source reputation persistent rather than judged fresh each time. It records from day one even though the weighting that reads it comes later — a correction-and-corroboration history can't be reconstructed after the fact. This is what turns "no one else's expert was taught by you" from a metaphor into a data structure. (Roadmap: G8 records it; P2.5 weights it.)