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.sx

@@ -433,15 +433,6 @@
       (run-post-render-hooks)
       (flush-collected-styles)
       (set-timeout (fn () (process-elements nil)) 0)
-      (dom-listen
-        (dom-window)
-        "popstate"
-        (fn
-          (e)
-          (let
-            ((state (host-get e "state"))
-              (scrollY (if state (or (get state "scrollY") 0) 0)))
-            (handle-popstate scrollY))))
       (dom-set-attr
         (host-get (dom-document) "documentElement")
         "data-sx-ready"