Fix stepper preview flash: render lake on client, screenshot-based test

Root cause: the lake had (when (not (client?)) ...) guard — SSR rendered
"the joy of sx" preview but client skipped it. replaceChildren swapped
in an empty lake. The rebuild-preview effect was skipped (first-run
optimization), so the preview stayed blank for ~500ms.

Fix: remove the client? guard so the lake renders on both server and
client. The template's steps-to-preview produces the initial preview.
The effect only fires on subsequent step changes (not first run).

Test: replaced MutationObserver approach with screenshot comparison.
Loads page with JS blocked (pure SSR), takes screenshot. Loads with JS
(hydration), takes screenshot. Compares pixels. Any visual difference
fails the test.

Result: "No visual flash: screenshots identical" — passes.

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

sx/sx/home-stepper.sx

@@ -400,6 +400,4 @@
               "▶"))
           (lake
             :id "home-preview"
-            (when
-              (not (client?))
-              (steps-to-preview (deref steps) (deref step-idx)))))))))
+            (steps-to-preview (deref steps) (deref step-idx))))))))