The resolver returns the first entry matching a declared name, so a
later duplicate within provides.templates or provides.scripts was
silently unreachable while still counted by ExtensionManifest
properties. Reject duplicates at manifest-validation time instead.
Also clarify EXTENSION-DEVELOPMENT-GUIDE.md's provides section: hooks
and events are top-level manifest fields, not provides sub-fields, so
the "at least one of ..." wording doesn't imply they can be nested
under provides.
