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>

src/betaflight.h

@@ -0,0 +1,17 @@
+#pragma once
+#include <chrono>
+#include <optional>
+#include <string>
+
+#include "serial.h"
+
+namespace bfebbx {
+
+// Pure: extract craft_name from captured CLI output.
+std::optional<std::string> parseCraftName(const std::string& cliOutput);
+
+void enterCli(Serial& s);
+std::optional<std::string> getCraftName(Serial& s, std::chrono::milliseconds timeout);
+void enterMsc(Serial& s);
+
+}  // namespace bfebbx