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-html.sx

@@ -85,6 +85,10 @@
               (= name "lake")
                 (render-html-lake args env)
 
+              ;; Marsh — reactive server-morphable slot within an island
+              (= name "marsh")
+                (render-html-marsh args env)
+
               ;; HTML tag
               (contains? HTML_TAGS name)
                 (render-html-element name args env)
@@ -334,6 +338,46 @@
            "</" lake-tag ">"))))
 
 
+;; --------------------------------------------------------------------------
+;; render-html-marsh — SSR rendering of a reactive server-morphable slot
+;; --------------------------------------------------------------------------
+;;
+;; (marsh :id "name" :tag "div" :transform fn children...)
+;; → <div data-sx-marsh="name">children</div>
+;;
+;; Like a lake but reactive: during morph, new content is parsed as SX and
+;; re-evaluated in the island's signal scope. Server renders children normally;
+;; the :transform is a client-only concern.
+
+(define render-html-marsh
+  (fn (args env)
+    (let ((marsh-id nil)
+          (marsh-tag "div")
+          (children (list)))
+      (reduce
+        (fn (state arg)
+          (let ((skip (get state "skip")))
+            (if skip
+              (assoc state "skip" false "i" (inc (get state "i")))
+              (if (and (= (type-of arg) "keyword")
+                       (< (inc (get state "i")) (len args)))
+                (let ((kname (keyword-name arg))
+                      (kval (trampoline (eval-expr (nth args (inc (get state "i"))) env))))
+                  (cond
+                    (= kname "id")        (set! marsh-id kval)
+                    (= kname "tag")       (set! marsh-tag kval)
+                    (= kname "transform") nil)
+                  (assoc state "skip" true "i" (inc (get state "i"))))
+                (do
+                  (append! children arg)
+                  (assoc state "i" (inc (get state "i"))))))))
+        (dict "i" 0 "skip" false)
+        args)
+      (str "<" marsh-tag " data-sx-marsh=\"" (escape-attr (or marsh-id "")) "\">"
+           (join "" (map (fn (c) (render-to-html c env)) children))
+           "</" marsh-tag ">"))))
+
+
 ;; --------------------------------------------------------------------------
 ;; render-html-island — SSR rendering of a reactive island
 ;; --------------------------------------------------------------------------