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_serial.cpp

@@ -0,0 +1,33 @@
+#include "../src/serial.h"
+#include <cassert>
+#include <chrono>
+#include <string>
+#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);
+
+    // Pre-load a canned response into the master's read buffer.
+    const char* resp = "hello world#";
+    assert(write(slave, resp, 12) == 12);
+
+    Serial s = Serial::fromFd(master);
+    std::string got = s.readUntil("#", 1000ms);
+    assert(got == "hello world#");
+
+    // Timeout path: nothing more to read, returns what it has within the window.
+    std::string none = s.readUntil("#", 100ms);
+    assert(none.empty());
+
+    close(slave);
+    return 0;
+}