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>

sx/sx/not-found.sx

@@ -0,0 +1,20 @@
+;; 404 Not Found page content
+
+(defcomp ~not-found-content (&key path)
+  (div :class "max-w-3xl mx-auto px-4 py-12 text-center"
+    (h1 :style (cssx (:text (colour "stone" 800) (size "3xl") (weight "bold")))
+      "404")
+    (p :class "mt-4"
+       :style (cssx (:text (colour "stone" 500) (size "lg")))
+      "Page not found")
+    (when path
+      (p :class "mt-2"
+         :style (cssx (:text (colour "stone" 400) (size "sm") (family "mono")))
+        path))
+    (a :href "/"
+       :sx-get "/" :sx-target "#main-panel" :sx-select "#main-panel"
+       :sx-swap "outerHTML" :sx-push-url "true"
+       :class "inline-block mt-6 px-4 py-2 rounded border transition-colors"
+       :style (cssx (:text (colour "violet" 700) (size "sm"))
+                    (:border (colour "violet" 200)))
+      "Back to home")))