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>

README.md

@@ -0,0 +1,78 @@
+# bfebbx — Betaflight Extract Blackbox
+
+Extract a Betaflight flight controller's blackbox log to a timestamped,
+craft-named file in a single command. macOS and Linux.
+
+## What it does
+
+1. Auto-detects the FC's serial port.
+2. Reads `craft_name` over the Betaflight CLI.
+3. Sends `msc` to reboot the FC into USB mass-storage mode.
+4. Waits for the storage volume to mount, then copies `btfl_all.bbl` to
+   `<craft_name>_<YYYYMMDD>_<HHMMSS>.bbl`.
+5. Ejects the volume.
+
+## Build
+
+```bash
+cmake -S . -B build
+cmake --build build
+ctest --test-dir build            # run unit tests
+```
+
+The binary is `build/bfebbx`.
+
+Requirements: a C++17 compiler and CMake ≥ 3.16. No third-party libraries.
+macOS links the system DiskArbitration/CoreFoundation frameworks.
+
+## Usage
+
+```
+bfebbx [--port PORT] [--output DIR] [--craft-name NAME] [--timeout SEC] [-v]
+```
+
+| Flag | Default | Meaning |
+|------|---------|---------|
+| `--port` | auto-detect | Serial device (e.g. `/dev/tty.usbmodem1101`). |
+| `--output` | current dir | Directory for the extracted `.bbl`. |
+| `--craft-name` | read from FC | Override / fallback craft name. |
+| `--timeout` | 30 | Seconds to wait for the storage volume. |
+| `-v` | off | Verbose progress on stderr. |
+
+### Exit codes
+
+| Code | Meaning |
+|------|---------|
+| 0 | Success |
+| 2 | No candidate serial port |
+| 3 | Multiple candidate ports (pass `--port`) |
+| 4 | Serial open / handshake failure |
+| 5 | Storage volume did not appear before timeout |
+| 6 | `btfl_all.bbl` not found on the volume |
+| 7 | Copy failed |
+| 8 | Destination file already exists |
+| 64 | Usage error |
+
+## Manual integration test (requires a real FC)
+
+The pure logic is unit-tested. The serial handshake and eject need hardware:
+
+1. Flash/confirm a Betaflight FC with blackbox logs on onboard flash.
+2. Connect it via USB. Confirm the port appears: `ls /dev/tty.usbmodem*`
+   (macOS) or `ls /dev/ttyACM*` (Linux).
+3. Run `./build/bfebbx -v` in an empty directory.
+4. Verify:
+   - The craft name is read and printed.
+   - The FC re-enumerates as a USB drive.
+   - A file `<craft_name>_<date>_<time>.bbl` is written and matches the size
+     of `btfl_all.bbl` on the drive.
+   - The volume is ejected (no longer in `/Volumes` on macOS / `/proc/mounts`
+     on Linux).
+5. Re-run in the same directory within the same second is not required; the
+   timestamp differs each run, so collisions are not expected. To exercise the
+   overwrite guard, `--craft-name X` twice within one second and confirm exit
+   code 8.
+
+## Design
+
+See `docs/superpowers/specs/2026-07-22-bfebbx-design.md`.