URL restructure, 404 page, trailing slash normalization, layout fixes

- Rename /reactive-islands/ → /reactive/, /reference/ → /hypermedia/reference/,
  /examples/ → /hypermedia/examples/ across all .sx and .py files
- Add 404 error page (not-found.sx) working on both server refresh and
  client-side SX navigation via orchestration.sx error response handling
- Add trailing slash redirect (GET only, excludes /api/, /static/, /internal/)
- Remove blue sky-500 header bar from SX docs layout (conditional on header-rows)
- Fix 405 on API endpoints from trailing slash redirect hitting POST/PUT/DELETE
- Fix client-side 404: orchestration.sx now swaps error response content
  instead of silently dropping it
- Add new plan files and home page component

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

shared/sx/async_eval.py

@@ -1340,6 +1340,10 @@ async def _aser(expr: Any, env: dict[str, Any], ctx: RequestContext) -> Any:
         if name == "lake":
             return await _aser_call(name, expr[1:], env, ctx)
 
+        # Marsh — serialize (reactive server-morphable slot within island)
+        if name == "marsh":
+            return await _aser_call(name, expr[1:], env, ctx)
+
         # HTML tag — serialize (don't render to HTML)
         if name in HTML_TAGS:
             return await _aser_call(name, expr[1:], env, ctx)