Revert render-dom-lake to original, simplify stepper lake

Reverted render-dom-lake SSR reuse — it broke OOB swaps (claimed
old lake elements during morph, stale content in copyright). The
framework's morphing handles lake updates correctly already.

Stepper: lake passes nil on client (prevents raw SX flash), effect
always calls rebuild-preview (no initial-render flag needed). Server
renders the expression for SSR; client rebuilds via render-to-dom
after boot when ~tw is available.

Removed initial-render dict flag — unnecessary complexity.

Copyright route not updating is a pre-existing issue: render-dom-island
renders the header island inline during OOB content rendering (sets
island-hydrated mark), but the copyright lake content doesn't reflect
the new path. Separate investigation needed.

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

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

@@ -346,7 +346,19 @@
         (resp-ok status get-header text)
         (when
           resp-ok
-          (dom-set-inner-html main text)
+          (let
+            ((ct (or (get-header "content-type") "")))
+            (if
+              (contains? ct "text/html")
+              (let
+                ((parser (host-new "DOMParser"))
+                  (doc (host-call parser "parseFromString" text "text/html"))
+                  (content (host-call doc "querySelector" "#sx-content")))
+                (if
+                  content
+                  (dom-set-inner-html main (host-get content "innerHTML"))
+                  (dom-set-inner-html main text)))
+              (dom-set-inner-html main text)))
           (post-swap main)
           (host-call (dom-window) "scrollTo" 0 scroll-y)))
       (fn (err) (log-warn (str "fetch-and-restore error: " err))))))