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>

docs/architecture.md

@@ -219,6 +219,14 @@ IPv6 requires special handling on macOS:
 - NDP probe packet sent before data blast to populate neighbor cache
 - Adaptive backoff on ENOBUFS (200μs→10ms, resets on success)
 
+### 11. CPU usage monitoring
+
+A background OS thread samples system CPU every 1 second via:
+- **macOS:** `host_statistics(HOST_CPU_LOAD_INFO)` — returns user/system/idle/nice ticks
+- **Linux:** `/proc/stat` — reads aggregate CPU line
+
+The percentage is stored in a global `AtomicU8` and included in every status message at byte 1 using MikroTik's encoding: `0x80 | percentage`. On receive, the remote CPU is decoded with `byte & 0x7F` and capped at 100%. Both local and remote CPU are displayed per interval and logged to CSV/syslog.
+
 ## File Layout
 
 ```
@@ -232,6 +240,7 @@ btest-rs/
 │   ├── server.rs            # Server mode: listener, TCP/UDP handlers
 │   ├── client.rs            # Client mode: connector, TCP/UDP handlers
 │   ├── bandwidth.rs         # Rate limiting, formatting, shared state
+│   ├── cpu.rs               # CPU usage sampler (macOS + Linux)
 │   ├── csv_output.rs        # CSV result logging (append-mode, auto-header)
 │   └── syslog_logger.rs     # Remote syslog sender (RFC 3164 / BSD format)
 ├── tests/