Keep the colleague's microbu-esp32c5 tree in this repository

obu-firmware builds against vanetza-idf from microbu-esp32c5/external, but
that tree was gitignored, so a clone of this repository could not build the
firmware it ships. It is now committed here as ordinary files in its own
folder, microbu-esp32c5/: the colleague's commit cf4b99f plus the V2X2MAP
bridge's signature verification (--trust) used on the bench. Nothing is
fetched from or pushed to the colleague's repository; this repository and
its remotes carry everything. The folder's own .gitignore keeps build output,
downloaded components and private key material out, as it did there; the
committed file set is identical to that repository's tracked files.

The ESP32-C5 is still flashed from obu-firmware/, which only takes
vanetza-idf from microbu-esp32c5/, so the two stay separate folders.
FLASHING.md says how to take a newer version of the colleague's tree (copy
it over the folder, rebuild, test, commit).

microbu-esp32c5/firmware/main/test_channel.hpp

@@ -0,0 +1,15 @@
+#pragma once
+// Test channel (serial frame type 0x11, test firmware only): software lower tester and the
+// diagnostic hooks the ETSI campaigns need. Not part of the phone interface, never on BLE.
+#include "link_protocol.hpp"
+#include "station.hpp"
+
+namespace microbu {
+
+/// @brief Executes one software-lower-tester request against the station.
+/// @param station Station to execute the request against.
+/// @param request Raw test-channel request bytes.
+/// @return Reply bytes: [result][record count]{[kind][length u16 LE][bytes]}.
+link::Bytes test_channel_execute(Station& station, const link::Bytes& request);
+
+} // namespace microbu