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_mounts.cpp

@@ -0,0 +1,29 @@
+#include "../src/mounts.h"
+#include <cassert>
+
+int main() {
+    using namespace bfebbx;
+
+    std::vector<MountEntry> before = {
+        {"/dev/disk1s1", "/"},
+        {"/dev/disk2s1", "/Volumes/Data"},
+    };
+    std::vector<MountEntry> after = before;
+    after.push_back({"/dev/disk5s1", "/Volumes/NO NAME"});
+
+    auto added = diffMounts(before, after);
+    assert(added.size() == 1);
+    assert(added[0].mountpoint == "/Volumes/NO NAME");
+    assert(added[0].device == "/dev/disk5s1");
+
+    // No change -> empty diff
+    assert(diffMounts(before, before).empty());
+
+    // listMounts should at least return the root filesystem on a live system
+    bool sawRoot = false;
+    for (const auto& m : listMounts())
+        if (m.mountpoint == "/") sawRoot = true;
+    assert(sawRoot);
+
+    return 0;
+}