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FLASHING.md says how to take a newer version of the colleague's tree (copy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

@@ -0,0 +1,79 @@
+#include "test_channel.hpp"
+#if CONFIG_MICROBU_TEST_CHANNEL
+#include <algorithm>
+
+namespace microbu {
+
+link::Bytes test_channel_execute(Station& station, const link::Bytes& request) {
+    using link::Code;
+    Code result = Code::malformed;
+    std::deque<TestRecord> records;
+    link::Bytes extra;
+    if (!request.empty()) {
+        link::Reader r(request, 1);
+        switch (request[0]) {
+        case 0x01: { // MIRROR mode
+            const auto mode = r.u8();
+            if (r.done() && mode <= 2) { station.test_mirror(static_cast<Station::Mirror>(mode)); result = Code::accepted; }
+            break;
+        }
+        case 0x02: { // INJECT source destination gnpdu
+            vanetza::MacAddress source, destination;
+            r.bytes(source.octets.data(), 6);
+            r.bytes(destination.octets.data(), 6);
+            auto gnpdu = r.rest();
+            if (r.ok() && !gnpdu.empty()) result = station.test_inject(source, destination, std::move(gnpdu));
+            break;
+        }
+        case 0x03: { // GN_REQUEST traffic class payload
+            const auto tc = r.u8();
+            auto payload = r.rest();
+            if (r.ok() && !payload.empty()) result = station.test_gn_request(tc, std::move(payload));
+            break;
+        }
+        case 0x04: { // DRAIN
+            bool overflow = false;
+            records = station.test_drain(overflow, 1536 - 2);
+            result = overflow ? Code::resource_limit : Code::accepted;
+            break;
+        }
+        case 0x05: // RESET
+            station.test_reset();
+            result = Code::accepted;
+            break;
+        case 0x06: { // RADIO_BURST [channel u16 LE][power_quarter_db u8][mcs u8][count u16 LE][interval_ms u16 LE][payload_len u16 LE]
+            const auto channel = r.u16();
+            const auto power_quarter_db = r.u8();
+            const auto mcs = r.u8();
+            const auto count = r.u16();
+            const auto interval_ms = r.u16();
+            const auto payload_len = r.u16();
+            if (r.done()) {
+                const double power_dbm = power_quarter_db / 4.0;
+                result = station.test_radio_burst(channel, power_dbm, mcs, count, interval_ms, payload_len);
+            }
+            break;
+        }
+        case 0x07: { // CCA_SAMPLE [duration_ms u16 LE] -- CCA polling feasibility probe
+            const auto duration_ms = r.u16();
+            if (r.done()) result = station.test_cca_sample(duration_ms, extra);
+            break;
+        }
+        default:
+            result = Code::unknown_opcode;
+        }
+    }
+    link::Writer w;
+    w.u8(static_cast<std::uint8_t>(result));
+    w.u8(static_cast<std::uint8_t>(std::min<std::size_t>(records.size(), 255)));
+    for (const auto& record : records) {
+        w.u8(record.kind);
+        w.u16(static_cast<std::uint16_t>(record.bytes.size()));
+        w.bytes(record.bytes);
+    }
+    w.bytes(extra);
+    return w.out;
+}
+
+} // namespace microbu
+#endif // CONFIG_MICROBU_TEST_CHANNEL