Updates from Gitea, step 1: the model (host-tested)

A streaming JSON scanner (a 33 KB list of releases costs a few hundred
bytes), the release reader built on it, HTTP response heads and chunked
bodies, URLs, and the decisions: which release is an update, whether to
announce it, and which download URLs the device takes. Tested against the
real answers of git.twis.la.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/release/src/update_check.cpp

@@ -0,0 +1,15 @@
+#include "update_check.h"
+
+#include "http_head.h"
+
+namespace roro::release {
+
+bool trustedAssetUrl(const std::string& url, const std::string& host, const std::string& repo) {
+    Url u = parseUrl(url);
+    if (!u.ok || !u.https || u.port != 443 || u.host != host) return false;
+    std::string prefix = "/" + repo + "/releases/download/";
+    return u.path.compare(0, prefix.size(), prefix) == 0 && u.path.find("..") == std::string::npos &&
+           u.path.find('?') == std::string::npos && u.path.find('#') == std::string::npos;
+}
+
+}  // namespace roro::release