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>

web/lib/boot-helpers.sx

@@ -589,13 +589,19 @@
 (define sx-render
   (fn (text)
     ;; Parse SX text and render to a DOM fragment.
+    ;; Islands are rendered as empty markers (span with data-sx-island)
+    ;; — post-swap will hydrate them. This matches the server's aser mode
+    ;; where island calls are serialized without expansion.
     (let ((doc (host-global "document"))
           (frag (host-call doc "createDocumentFragment"))
           (exprs (sx-parse text)))
+      ;; Push marker mode: render-dom-island creates markers, not full renders
+      (scope-push! "sx-render-markers" true)
       (for-each (fn (expr)
         (let ((result (render-to-dom expr (get-render-env nil) nil)))
           (when result (dom-append frag result))))
         exprs)
+      (scope-pop! "sx-render-markers")
       frag)))
 
 (define sx-hydrate