Fix bridge newline escaping, stepper optimization, dom-listen dedup

- ocaml_sync.py: escape newlines in eval/load_source to prevent
  protocol desync (bridge crashed on any multi-line SX)
- Stepper: do-back uses rebuild-preview (O(1) render) instead of
  replaying all steps. Hydration effect same. Cookie save on button
  click only.
- dom.sx: remove duplicate dom-listen (was shadowing the one at
  line 351 that adapter-dom.sx's dom-on wraps)
- orchestration.sx: fix bind-sse-swap close paren count
- safe_eq: Dict equality via __host_handle for DOM node identity

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

web/lib/dom.sx

@@ -97,10 +97,6 @@
   (fn (child parent)
     (and child parent (host-call parent "contains" child))))
 
-(define dom-listen
-  (fn (el event-name handler)
-    (host-call el "addEventListener" event-name (host-callback handler))))
-
 (define dom-attr-list
   (fn (el)
     ;; Return list of (name value) pairs for all attributes on the element.