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/adapter-dom.sx

@@ -630,7 +630,7 @@
             (env-set! local "children" child-frag)))
 
         ;; Create the island container element
-        (let ((container (dom-create-element "div" nil))
+        (let ((container (dom-create-element "span" nil))
               (disposers (list)))
 
           ;; Mark as island