Fix stepper: lake SSR preservation + stack rebuild after stepping

Three fixes:

1. Framework: render-dom-lake preserves SSR elements during hydration.
   When client-side render-to-dom encounters a lake with an existing
   DOM element (from SSR), it reuses that element instead of creating
   a new one. This prevents the SSR HTML from being replaced with
   unresolvable raw SX expressions (~tw calls).

2. Stepper: skip rebuild-preview on initial hydration. Uses a non-
   reactive dict flag (not a signal) to avoid triggering the effect
   twice. On first run, just initializes the DOM stack from the
   existing SSR content by computing open-element depth from step
   types and walking lastElementChild.

3. Stepper: rebuild-preview computes correct DOM stack after re-render.
   Same depth computation + DOM walk approach. This fixes the bug where
   do-step after do-back would append elements to the wrong parent
   (e.g. "sx" span outside h1).

Also: increased code view font-size from 0.5rem to 0.85rem.

Playwright tests:
- lake never shows raw SX during hydration (mutation observer)
- back 6 + forward 6 keeps all 4 spans inside h1

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

web/adapter-dom.sx

@@ -920,12 +920,17 @@
         (dict "i" 0 "skip" false)
         args)
       (let
-        ((el (dom-create-element lake-tag nil)))
-        (dom-set-attr el "data-sx-lake" (or lake-id ""))
-        (for-each
-          (fn (c) (dom-append el (render-to-dom c env ns)))
-          children)
-        el))))
+        ((existing (when (and (client?) lake-id) (dom-query (str "[data-sx-lake=\"" lake-id "\"]")))))
+        (if
+          existing
+          existing
+          (let
+            ((el (dom-create-element lake-tag nil)))
+            (dom-set-attr el "data-sx-lake" (or lake-id ""))
+            (for-each
+              (fn (c) (dom-append el (render-to-dom c env ns)))
+              children)
+            el))))))
 
 (define
   render-dom-marsh