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/signals.sx

@@ -306,7 +306,47 @@
 
 
 ;; ==========================================================================
-;; 12. Named stores — page-level signal containers (L3)
+;; 12. Marsh scopes — child scopes within islands
+;; ==========================================================================
+;;
+;; Marshes are zones inside islands where server content is re-evaluated
+;; in the island's reactive context. When a marsh is re-morphed with new
+;; content, its old effects and computeds must be disposed WITHOUT disturbing
+;; the island's own reactive graph.
+;;
+;; Scope hierarchy: island → marsh → effects/computeds
+;; Disposing a marsh disposes its subscope. Disposing an island disposes
+;; all its marshes. The signal graph is a tree, not a flat list.
+;;
+;; Platform interface required:
+;;   (dom-set-data el key val) → void  — store JS value on element
+;;   (dom-get-data el key)     → any   — retrieve stored value
+
+(define with-marsh-scope
+  (fn (marsh-el body-fn)
+    ;; Execute body-fn collecting all disposables into a marsh-local list.
+    ;; Nested under the current island scope — if the island is disposed,
+    ;; the marsh is disposed too (because island scope collected the marsh's
+    ;; own dispose function).
+    (let ((disposers (list)))
+      (with-island-scope
+        (fn (d) (append! disposers d))
+        body-fn)
+      ;; Store disposers on the marsh element for later cleanup
+      (dom-set-data marsh-el "sx-marsh-disposers" disposers))))
+
+(define dispose-marsh-scope
+  (fn (marsh-el)
+    ;; Dispose all effects/computeds registered in this marsh's scope.
+    ;; Parent island scope and sibling marshes are unaffected.
+    (let ((disposers (dom-get-data marsh-el "sx-marsh-disposers")))
+      (when disposers
+        (for-each (fn (d) (invoke d)) disposers)
+        (dom-set-data marsh-el "sx-marsh-disposers" nil)))))
+
+
+;; ==========================================================================
+;; 13. Named stores — page-level signal containers (L3)
 ;; ==========================================================================
 ;;
 ;; Stores persist across island creation/destruction. They live at page