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/orchestration.sx

@@ -296,11 +296,11 @@
                 ;; Hydrated islands are preserved — the morph algorithm
                 ;; keeps their live signals and only morphs their lakes.
                 (dispose-islands-in target)
-                ;; Swap
+                ;; Swap — swap-dom-nodes returns the new element for outerHTML
                 (with-transition use-transition
                   (fn ()
-                    (swap-dom-nodes target content swap-style)
-                    (post-swap target)))))))))))
+                    (let ((swap-result (swap-dom-nodes target content swap-style)))
+                      (post-swap (or swap-result target))))))))))))
 
 
 (define handle-html-response :effects [mutation io]