Implement bfebbx: Betaflight blackbox extractor

Cross-platform (macOS + Linux) C++17 CLI that extracts a Betaflight FC's
blackbox log in one command: auto-detect the serial port, read craft_name
over the Betaflight CLI, send `msc` to reboot into USB mass storage, wait
for the volume to mount, copy btfl_all.bbl to
<craft_name>_<YYYYMMDD>_<HHMMSS>.bbl, and eject.

Native throughout (termios serial, getmntinfo/proc-mounts mount detection,
DiskArbitration/umount eject); the only subprocess is the Linux udisksctl
eject fallback. Includes CMake/ctest build, unit tests for the pure logic
(naming, craft_name parsing, mount diff, serial via pty, CLI parsing),
README with a manual hardware integration procedure, and design/plan docs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_betaflight_proto.cpp

@@ -0,0 +1,38 @@
+#include "../src/betaflight.h"
+#include "../src/serial.h"
+#include <cassert>
+#include <chrono>
+#include <termios.h>
+#include <unistd.h>
+#if defined(__APPLE__)
+#include <util.h>
+#else
+#include <pty.h>
+#endif
+
+int main() {
+    using namespace bfebbx;
+    using namespace std::chrono_literals;
+
+    int master = -1, slave = -1;
+    assert(openpty(&master, &slave, nullptr, nullptr, nullptr) == 0);
+
+    // Set slave to raw mode to disable line-ending translation
+    struct termios tio;
+    assert(tcgetattr(slave, &tio) == 0);
+    cfmakeraw(&tio);
+    assert(tcsetattr(slave, TCSANOW, &tio) == 0);
+
+    // FC echoes the command then reports the value and a prompt.
+    const char* canned = "# get craft_name\r\ncraft_name = TESTQUAD\r\n# ";
+    ssize_t len = static_cast<ssize_t>(std::string(canned).size());
+    assert(write(slave, canned, static_cast<size_t>(len)) == len);
+
+    Serial s = Serial::fromFd(master);
+    auto name = getCraftName(s, 1000ms);
+    assert(name.has_value());
+    assert(name.value() == "TESTQUAD");
+
+    close(slave);
+    return 0;
+}