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>

shared/static/wasm/sx/page-helpers.sx

@@ -49,7 +49,7 @@
                     "Other")))
               (when
                 (not (has-key? categories category))
-                (dict-set! categories category (mutable-list)))
+                (dict-set! categories category (list)))
               (append! (get categories category) {:doc (or (get kwargs "doc") "") :example (or (get kwargs "example") "") :tail-position (or (get kwargs "tail-position") "") :syntax (or (get kwargs "syntax") "") :name name}))))
         parsed-exprs)
       categories)))
@@ -200,7 +200,7 @@
   (fn
     ((pages-raw :as list))
     (let
-      ((pages-data (mutable-list)) (client-count 0) (server-count 0))
+      ((pages-data (list)) (client-count 0) (server-count 0))
       (for-each
         (fn
           ((page :as dict))