Reactive island preservation across server-driven morphs

Islands survive hypermedia swaps: morph-node skips hydrated
data-sx-island elements when the same island exists in new content.
dispose-islands-in skips hydrated islands to prevent premature cleanup.

- @client directive: .sx files marked ;; @client send define forms to browser
- CSSX client-side: cssxgroup renamed (no hyphen) to avoid isRenderExpr
  matching it as a custom element — was producing [object HTMLElement]
- Island wrappers: div→span to avoid block-in-inline HTML parse breakage
- ~sx-header is now a defisland with inline reactive colour cycling
- bootstrap_js.py defaults output to shared/static/scripts/sx-browser.js
- Deleted stale sx-ref.js (sx-browser.js is the canonical browser build)
- Hegelian Synthesis essay: dialectic of hypertext and reactivity
- component-source helper handles Island types for docs pretty-printing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/engine.sx

@@ -338,6 +338,16 @@
           (dom-has-attr? old-node "sx-ignore"))
         nil
 
+      ;; Hydrated island → preserve reactive state across server swaps.
+      ;; If old and new are the same island (by name), keep the old DOM
+      ;; with its live signals, effects, and event listeners intact.
+      (and (dom-has-attr? old-node "data-sx-island")
+           (is-processed? old-node "island-hydrated")
+           (dom-has-attr? new-node "data-sx-island")
+           (= (dom-get-attr old-node "data-sx-island")
+              (dom-get-attr new-node "data-sx-island")))
+        nil
+
       ;; Different node type or tag → replace wholesale
       (or (not (= (dom-node-type old-node) (dom-node-type new-node)))
           (not (= (dom-node-name old-node) (dom-node-name new-node))))