Answer architecture round 1: one owner keeps a folder's loaded and unloaded names apart, each loaded category is checked against its index entry, addSource returns what it replaced, the manifest cache goes, ErrLoad exits 1, and the docs say what New and Fake now refuse

AGENTS.md

@@ -47,7 +47,7 @@ In [docs/decisions.md](docs/decisions.md):
 - `FakeTemplate` and `NewTemplate` both stay
 - The shipped data is embedded, not discovered
 - The shipped data is Go packages a library imports by choice, and the CLI carries every one, from 0.1.0
-- A source whose manifest carries an index loads each category on first reach; any other loads in `New`, with what it reads and every indexed category reaching what it replaces
+- A source whose manifest carries an index loads each category on first reach; any other loads in `New`, with what it reads and every indexed category depending on what it provides or a later source replaced
 - Every `{…}` draws afresh, and only a name keeps a pick
 - Reference sigils follow the filesystem
 - A change to what exists is a major; a minor only adds