Answer architecture round 3: New loads only sources without an index and what they read, an entry drops reads, ErrLoad sits beside New, the manifest decoder moves to manifest.go and names each mistake once as jsonvalue.Kind words it

internal/jsonvalue/jsonvalue.go

@@ -4,6 +4,7 @@ package jsonvalue
 import (
 	"bytes"
 	"encoding/json"
+	"fmt"
 )
 
 // Value is a JSON value that only Decode, String and Object make, so a template never compiles
@@ -37,3 +38,22 @@ func Object(fields map[string]Value) Value {
 // Any is the value as encoding/json decodes it: a map[string]any, []any, string, json.Number,
 // bool or nil.
 func (v Value) Any() any { return v.v }
+
+// Kind names the kind of a value as Any gives it, such as "a number" or "a list".
+func Kind(v any) string {
+	switch v.(type) {
+	case json.Number:
+		return "a number"
+	case bool:
+		return "a boolean"
+	case string:
+		return "a string"
+	case []any:
+		return "a list"
+	case map[string]any:
+		return "an object"
+	case nil:
+		return "null"
+	}
+	return fmt.Sprintf("%T", v)
+}