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/templates/layout.sx

@@ -1,15 +1,16 @@
 (defcomp ~app-body (&key header-rows filter aside menu content)
   (div :class "max-w-screen-2xl mx-auto py-1 px-1"
-    (div :class "w-full"
-      (details :class "group/root p-2" :data-toggle-group "mobile-panels"
-        (summary
-          (header :class "z-50"
-            (div :id "root-header-summary"
-                 :class "flex items-start gap-2 p-1 bg-sky-500"
-              (div :id "root-header-child" :class "flex flex-col w-full items-center"
-                (when header-rows header-rows)))))
-        (div :id "root-menu" :sx-swap-oob "outerHTML" :class "md:hidden"
-          (when menu menu))))
+    (when header-rows
+      (div :class "w-full"
+        (details :class "group/root p-2" :data-toggle-group "mobile-panels"
+          (summary
+            (header :class "z-50"
+              (div :id "root-header-summary"
+                   :class "flex items-start gap-2 p-1 bg-sky-500"
+                (div :id "root-header-child" :class "flex flex-col w-full items-center"
+                  header-rows))))
+          (div :id "root-menu" :sx-swap-oob "outerHTML" :class "md:hidden"
+            (when menu menu)))))
     (div :id "filter"
       (when filter filter))
     (main :id "root-panel" :class "max-w-full"