Answer architecture round 2: each category site carries its source's default modules, set where the walk or the index loads it, and the note is written beside the resolve that finds the read; the decision states why New checks no reads

resolve.go

@@ -3,26 +3,18 @@ package fejkdata
 import (
 	"errors"
 	"fmt"
+	"strings"
 )
 
-// templateSite is a template to resolve, with its folder and its category's path, both empty for an
+// templateSite is a template to resolve, with its folder and its category, both empty for an
 // inline template, and eachNode's label for it.
 type templateSite struct {
 	t        *template
 	folder   []string
 	label    string
-	category string
+	category *categorySite
 }
 
-// unresolvedRead is a reference of a category's template naming nothing.
-type unresolvedRead struct {
-	category string
-	err      error
-}
-
-func (u unresolvedRead) Error() string { return u.err.Error() }
-func (u unresolvedRead) Unwrap() error { return u.err }
-
 // resolveCategoryTemplates resolves every template of the categories, once all data is merged, so a
 // reference sees the override-resolved tree.
 func resolveCategoryTemplates(sites []categorySite, root map[string]node) error {
@@ -30,7 +22,7 @@ func resolveCategoryTemplates(sites []categorySite, root map[string]node) error
 	for _, s := range sites {
 		if err := eachNode(s.n, s.path, func(label string, n node) error {
 			if t, isTemplate := n.(*template); isTemplate {
-				ts = append(ts, templateSite{t: t, folder: s.dir, label: label, category: s.path})
+				ts = append(ts, templateSite{t: t, folder: s.dir, label: label, category: &s})
 			}
 			return nil
 		}); err != nil {
@@ -66,12 +58,8 @@ type resolvedNames struct {
 func resolveTemplates(ts []templateSite, root map[string]node) error {
 	for _, s := range ts {
 		refs, err := s.t.resolveRefs(s.folder, s.label, root)
-		var ne noEntry
-		if err != nil && s.category != "" && errors.As(err, &ne) {
-			return unresolvedRead{category: s.category, err: err}
-		}
 		if err != nil {
-			return err
+			return s.withDefaultModules(err)
 		}
 		s.t.refs = refs
 	}
@@ -102,3 +90,16 @@ func resolveTemplates(ts []templateSite, root map[string]node) error {
 	}
 	return nil
 }
+
+// withDefaultModules adds to err, where a reference of a category names nothing, the modules
+// the category's source reads by default, or that it names none.
+func (s templateSite) withDefaultModules(err error) error {
+	var ne noEntry
+	if s.category == nil || !errors.As(err, &ne) {
+		return err
+	}
+	if modules := s.category.defaultModules; len(modules) > 0 {
+		return fmt.Errorf("%w; the module holding %s reads by default %s", err, s.category.path, strings.Join(modules, ", "))
+	}
+	return fmt.Errorf("%w, and the module holding %s names no module it reads by default", err, s.category.path)
+}