OK the only extends that works is without the theme name

whatever, can figure it out later.  I'm thinking extends is a bad pattern; the including
component templates thing seems much better.

agaric/templates/paragraph/paragraph--slide--columnar.html.twig

@@ -1,2 +1,2 @@
-{% extends "@agarica/paragraph/paragraph--slide.html.twig" %}
+{% extends "paragraph--slide.html.twig" %}
 {# We do this to "un-override" the columnar paragraph in Octavia. #}