v0.6.0: CPU monitoring, CSV with CPU, docs update, cleanup

New in v0.6.0:
- CPU usage: local/remote shown per interval (cpu: 12%/33%)
- Warning indicator (!) when CPU > 70% on either side
- MikroTik CPU encoding: 0x80 | percentage in status byte 1
- CSV includes local_cpu_pct and remote_cpu_pct columns
- Status message format corrected to match MikroTik wire format:
  [type:1][cpu:1][00:2][seq:4 LE][bytes:4 LE]
- Removed btest-opensource submodule (fully reimplemented)
- Deleted research/ecsrp5 branch
- Updated all docs: architecture, user-guide, man page, protocol
- Version bumped to 0.6.0

58 tests, all passing. Zero warnings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -11,6 +11,7 @@ A Rust reimplementation of the [MikroTik Bandwidth Test (btest)](https://wiki.mi
 - **Bidirectional testing** -- simultaneous upload and download
 - **Syslog logging** -- send structured events (auth, test start/end) to a remote syslog server
 - **CSV output** -- append machine-readable test results to a CSV file
+- **CPU usage monitoring** -- local and remote CPU shown per interval, warning at >70%
 - **Timed tests** -- `--duration` flag to automatically stop after N seconds
 - **Quiet mode** -- suppress terminal output for scripted/automated use
 - **NAT traversal** -- probe packet to open firewall holes for UDP receive