handler-source throws on missing handler, CLAUDE.md dev domains

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -349,6 +349,9 @@ Shared components live in `shared/sx/templates/` and are loaded by `load_shared_
 | relations | (internal only) | 8008 |
 | likes | (internal only) | 8009 |
 | orders | orders.rose-ash.com | 8010 |
+| sx_docs | sx.rose-ash.com | 8013 |
+
+**Dev serves live domains.** Docker dev containers bind-mount host files and Caddy routes public domains (e.g. `sx.rose-ash.com`) to the dev container ports (e.g. `localhost:8013`). There is no separate "local" vs "production" — editing files on the host and restarting the container updates the live site immediately. Playwright tests at `localhost:8013` test the same server visitors see at `sx.rose-ash.com`.
 
 ## Dev Container Mounts
 

sx/sx/data/helpers.sx

@@ -28,7 +28,7 @@
       (if
         (not (nil? val))
         (pretty-print val)
-        (str ";;; Handler not found: " name)))))
+        (error (str "Handler not found: " name))))))
 
 (define _spec-dirs (list "spec" "web" "shared/sx/ref" "lib"))