Add CPU usage monitoring, remove btest-opensource submodule

CPU usage feature:
- New cpu.rs module: background sampler thread, cross-platform (macOS + Linux)
- Status message byte 1 now carries CPU load (0-100%), matching MikroTik format
- Status format corrected: [type][cpu][00][00][seq:4 LE][bytes:4 LE]
- Client and server exchange CPU in every status message
- Display format: "cpu: 40%/12%" (local/remote), "!" warning if > 70%
- Both client and server show local + remote CPU per interval
- Syslog TEST_END could include CPU averages (future enhancement)

Removed btest-opensource submodule — we've fully reimplemented the protocol
with EC-SRP5 auth, multi-connection, IPv6, syslog, CSV, and CPU monitoring.
The original project is still credited in LICENSE and README.

58 tests, all passing.

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

src/lib.rs

@@ -1,7 +1,8 @@
 pub mod auth;
-pub mod csv_output;
 pub mod bandwidth;
 pub mod client;
+pub mod cpu;
+pub mod csv_output;
 pub mod ecsrp5;
 pub mod protocol;
 pub mod server;