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/ref/adapter-sx.sx

@@ -70,6 +70,10 @@
             (= name "lake")
               (aser-call name args env)
 
+            ;; Marsh — serialize (reactive server-morphable slot)
+            (= name "marsh")
+              (aser-call name args env)
+
             ;; HTML tag — serialize
             (contains? HTML_TAGS name)
               (aser-call name args env)