Fix island reactivity lost on client-side navigation; add Playwright tests

When morphing DOM after server fetch, the morph engine reuses elements
with the same tag. If old element was island A and new is island B,
syncAttrs updates data-sx-island but the JS property _sxBoundisland-hydrated
persists on the reused element. sx-hydrate-islands then skips it.

Fix: in morphNode, when data-sx-island attribute changes between old and
new elements, dispose the old island's signals and clear the hydration
flag so the new island gets properly hydrated.

New Playwright tests:
- counter → temperature navigation: temperature signals work
- temperature → counter navigation: counter signals work
- Direct load verification for both islands
- No JS errors during navigation

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

web/engine.sx

@@ -368,6 +368,15 @@
       ;; Element nodes → sync attributes, then recurse children
       (= (dom-node-type old-node) 1)
         (do
+          ;; If the island name changed, clear hydration flag so the new
+          ;; island gets re-hydrated after morph. The DOM element is reused
+          ;; (same tag) but the island content is completely different.
+          (when (and (dom-has-attr? old-node "data-sx-island")
+                     (dom-has-attr? new-node "data-sx-island")
+                     (not (= (dom-get-attr old-node "data-sx-island")
+                             (dom-get-attr new-node "data-sx-island"))))
+            (dispose-islands-in old-node)
+            (clear-processed! old-node "island-hydrated"))
           (sync-attrs old-node new-node)
           ;; Skip morphing focused input to preserve user's in-progress edits
           (when (not (and (dom-is-active-element? old-node)