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/html.py

@@ -501,6 +501,41 @@ def _render_lake(args: list, env: dict[str, Any]) -> str:
     return f'<{lake_tag} data-sx-lake="{_escape_attr(lake_id)}">{body}</{lake_tag}>'
 
 
+def _render_marsh(args: list, env: dict[str, Any]) -> str:
+    """Render a reactive server-morphable marsh slot.
+
+    (marsh :id "name" :tag "div" :transform fn children...)
+    → <div data-sx-marsh="name">children</div>
+
+    Marshes are zones where reactivity and hypermedia interpenetrate.
+    Like lakes but content is parsed as SX on the client and re-evaluated
+    in the island's signal scope. :transform is consumed but not used
+    server-side (it's a client-side concern).
+    """
+    marsh_id = ""
+    marsh_tag = "div"
+    children: list[Any] = []
+    i = 0
+    while i < len(args):
+        arg = args[i]
+        if isinstance(arg, Keyword) and i + 1 < len(args):
+            kname = arg.name
+            kval = _eval(args[i + 1], env)
+            if kname == "id":
+                marsh_id = str(kval) if kval is not None and kval is not NIL else ""
+            elif kname == "tag":
+                marsh_tag = str(kval) if kval is not None and kval is not NIL else "div"
+            elif kname == "transform":
+                pass  # Client-side only; skip
+            i += 2
+        else:
+            children.append(arg)
+            i += 1
+
+    body = "".join(_render(c, env) for c in children)
+    return f'<{marsh_tag} data-sx-marsh="{_escape_attr(marsh_id)}">{body}</{marsh_tag}>'
+
+
 def _render_list(expr: list, env: dict[str, Any]) -> str:
     """Render a list expression — could be an HTML element, special form,
     component call, or data list."""
@@ -530,6 +565,10 @@ def _render_list(expr: list, env: dict[str, Any]) -> str:
         if name == "lake":
             return _render_lake(expr[1:], env)
 
+        # --- marsh → reactive server-morphable slot within island --------
+        if name == "marsh":
+            return _render_marsh(expr[1:], env)
+
         # --- html: prefix → force tag rendering --------------------------
         if name.startswith("html:"):
             return _render_element(name[5:], expr[1:], env)