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FLASHING.md says how to take a newer version of the colleague's tree (copy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).

obu-firmware/CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.22)
 # obu-firmware: the ESP32-C5 half of the MicrOBU station, on the vanetza-idf C-ITS stack.
 #
 # Since 2026-09-23 this is a port of the colleague's standalone VRU station
-# (microbu-esp32c5/firmware, its own git repository beside this one and gitignored here). From it:
+# (microbu-esp32c5/firmware; a copy of that repository is kept in microbu-esp32c5/). From it:
 # BTP/GeoNetworking and the TS 103 097 security entity (vanetza-idf), the station-link message
 # layer over native USB Serial/JTAG and BLE GATT, the NVS credential store, and the C5 radio adapter.
 # Added here for this project: the raw receive path of the previous firmware (gn_unwrap.c, forwarded
@@ -14,14 +14,15 @@ cmake_minimum_required(VERSION 3.22)
 # by vanetza-idf's radio_c5.cmake and has only been validated there. The previous C firmware was
 # built with IDF 6.1; see FLASHING.md for the export script of each.
 #
-# vanetza-idf is taken from the colleague's tree rather than vendored (it is ~90 MB). Override with
-# -DVANETZA_IDF_DIR=... if it lives elsewhere.
+# vanetza-idf is taken from the colleague's tree in microbu-esp32c5/ rather than a second copy here
+# (it is ~70 MB). Override with -DVANETZA_IDF_DIR=... if it lives elsewhere.
 if(NOT VANETZA_IDF_DIR)
     set(VANETZA_IDF_DIR "${CMAKE_CURRENT_LIST_DIR}/../microbu-esp32c5/external/vanetza-idf")
 endif()
 if(NOT EXISTS "${VANETZA_IDF_DIR}/idf_component.yml")
-    message(FATAL_ERROR "vanetza-idf not found at ${VANETZA_IDF_DIR}: clone microbu-esp32c5 beside this "
-                        "repository or pass -DVANETZA_IDF_DIR=<path to external/vanetza-idf>")
+    message(FATAL_ERROR "vanetza-idf not found at ${VANETZA_IDF_DIR}: microbu-esp32c5/ is part of "
+                        "this repository; check it is complete, or pass "
+                        "-DVANETZA_IDF_DIR=<path to external/vanetza-idf>")
 endif()
 
 # Provenance guard for main/otm_tx_custom.c, copied unchanged from microbu-esp32c5/firmware/CMakeLists.txt: