Fix navigation: outerHTML swap, island markers, host handle equality, dispose

Navigation pipeline now works end-to-end:
- outerHTML swap uses dom-replace-child instead of morph-node (morph has
  a CEK continuation issue with nested for-each that needs separate fix)
- swap-dom-nodes returns the new element for outerHTML so post-swap
  hydrates the correct (new) DOM, not the detached old element
- sx-render uses marker mode: islands rendered as empty span[data-sx-island]
  markers, hydrated by post-swap. Prevents duplicate content from island
  body expansion + SX response nav rows.
- dispose-island (singular) called on old island before morph, not just
  dispose-islands-in (which only disposes sub-islands)

OCaml runtime:
- safe_eq: Dict equality checks __host_handle for DOM node identity
  (js_to_value creates new Dict wrappers per call, breaking physical ==)
- contains?: same host handle check
- to_string: trampoline thunks (fixes <thunk> display)
- as_number: trampoline thunks (fixes arithmetic on leaked thunks)

DOM platform:
- dom-remove, dom-attr-list (name/value pairs), dom-child-list (SX list),
  dom-is-active-element?, dom-is-input-element?, dom-is-child-of?, dom-on

All 5 reactive-nav Playwright tests pass.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -156,7 +156,12 @@ let () =
       (List lb | ListRef { contents = lb }) ->
       List.length la = List.length lb &&
       List.for_all2 safe_eq la lb
-    (* Dict/Lambda/Component/Island/Signal/NativeFn: physical only *)
+    (* Dict: check __host_handle for DOM node identity *)
+    | Dict a, Dict b ->
+      (match Hashtbl.find_opt a "__host_handle", Hashtbl.find_opt b "__host_handle" with
+       | Some (Number ha), Some (Number hb) -> ha = hb
+       | _ -> false)
+    (* Lambda/Component/Island/Signal/NativeFn: physical only *)
     | _ -> false
   in
   register "=" (fun args ->
@@ -430,6 +435,10 @@ let () =
          | Nil, Nil -> true
          | Symbol x, Symbol y -> x = y
          | Keyword x, Keyword y -> x = y
+         | Dict a, Dict b ->
+           (match Hashtbl.find_opt a "__host_handle", Hashtbl.find_opt b "__host_handle" with
+            | Some (Number ha), Some (Number hb) -> ha = hb
+            | _ -> false)
          | _ -> false)
       in
       Bool (List.exists (fun x -> safe_eq x item) l)