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/adapter-dom.sx

@@ -151,7 +151,14 @@
               (and (starts-with? name "~")
                    (env-has? env name)
                    (island? (env-get env name)))
-                (render-dom-island (env-get env name) args env ns)
+                ;; In marker mode (sx-render for swap), create empty marker span
+                ;; that post-swap will hydrate. Otherwise, full render.
+                (if (scope-peek "sx-render-markers")
+                  (let ((island (env-get env name))
+                        (marker (dom-create-element "span" nil)))
+                    (dom-set-attr marker "data-sx-island" (component-name island))
+                    marker)
+                  (render-dom-island (env-get env name) args env ns))
 
               ;; Component (~name)
               (starts-with? name "~")