feat: add precise diagnostics and optional client copy logging controls

internal/configdiag/error.go

@@ -0,0 +1,34 @@
+// Package configdiag carries value-free field diagnostics between framework
+// modules without making them depend on the application's config package.
+package configdiag
+
+import "errors"
+
+type Error struct {
+	Field, Reason string
+	cause         error
+}
+
+func (e *Error) Error() string       { return e.Field + ": " + e.Reason }
+func (e *Error) Unwrap() error       { return e.cause }
+func New(field, reason string) error { return &Error{Field: field, Reason: reason} }
+func Wrap(field, reason string, cause error) error {
+	return &Error{Field: field, Reason: reason, cause: cause}
+}
+func At(prefix string, err error) error {
+	if err == nil {
+		return nil
+	}
+	var d *Error
+	if errors.As(err, &d) {
+		field := prefix
+		if d.Field != "" {
+			if field != "" {
+				field += "."
+			}
+			field += d.Field
+		}
+		return Wrap(field, d.Reason, err)
+	}
+	return Wrap(prefix, "validation failed", err)
+}