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/html.py

@@ -414,7 +414,7 @@ def _render_component(comp: Component, args: list, env: dict[str, Any]) -> str:
 def _render_island(island: Island, args: list, env: dict[str, Any]) -> str:
     """Render an island as static HTML with hydration attributes.
 
-    Produces: <div data-sx-island="name" data-sx-state='{"k":"v",...}'>body HTML</div>
+    Produces: <span data-sx-island="name" data-sx-state='{"k":"v",...}'>body HTML</span>
     The client hydrates this into a reactive island.
     """
     import json as _json
@@ -460,12 +460,12 @@ def _render_island(island: Island, args: list, env: dict[str, Any]) -> str:
     state_json = _escape_attr(_json.dumps(state, separators=(",", ":"))) if state else ""
     island_name = _escape_attr(island.name)
 
-    parts = [f'<div data-sx-island="{island_name}"']
+    parts = [f'<span data-sx-island="{island_name}"']
     if state_json:
         parts.append(f' data-sx-state="{state_json}"')
     parts.append(">")
     parts.append(body_html)
-    parts.append("</div>")
+    parts.append("</span>")
     return "".join(parts)