fix: harden UCE runtime and starter

site/doc/pages/route_path_sanitize.txt

@@ -0,0 +1,34 @@
+:title
+route_path_sanitize
+
+:sig
+String route_path_sanitize(String path, String default_path = "index")
+
+:see
+route_path_is_safe
+route_path_normalize
+request_query_path
+request_query_route
+request_context_params
+
+:content
+Normalizes and validates a route path for file-backed routing.
+
+Rules:
+
+- leading and trailing `/` are removed
+- an empty path becomes `default_path`
+- every path segment must be non-empty
+- `.` and `..` segments are rejected
+- only ASCII letters, digits, `_`, and `-` are accepted inside a segment
+- unsafe input returns an empty string
+
+Use this instead of manually checking app routes before composing file paths.
+
+```cpp
+route_path_sanitize("/workspace/projects/"); // "workspace/projects"
+route_path_sanitize("../secret");            // ""
+route_path_sanitize("");                     // "index"
+```
+
+`request_query_path()`, `request_query_route()`, and the runtime-populated `ROUTE_PATH` params already use this sanitizer.