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/user-guide.md

@@ -235,10 +235,11 @@ btest -c 192.168.88.1 -r -P 3000
 ## Reading the Output
 
 ```
-[   1]  TX  264.50 Mbps (33062912 bytes)
-[   2]  TX  263.98 Mbps (32997376 bytes)
-[   2]  RX  263.98 Mbps (32997012 bytes)
-[   3]  RX  430.51 Mbps (53813376 bytes)  lost: 5
+[   1]  TX  264.50 Mbps (33062912 bytes)  cpu: 12%/0%
+[   2]  TX  263.98 Mbps (32997376 bytes)  cpu: 15%/33%
+[   2]  RX  263.98 Mbps (32997012 bytes)  cpu: 15%/33%
+[   3]  RX  430.51 Mbps (53813376 bytes)  lost: 5  cpu: 18%/45%
+[   4]  RX  450.00 Mbps (56250000 bytes)  cpu: 72%/85% !
 ```
 
 | Field | Meaning |
@@ -249,6 +250,8 @@ btest -c 192.168.88.1 -r -P 3000
 | `Mbps` | Megabits per second |
 | `bytes` | Raw bytes transferred in this interval |
 | `lost: N` | UDP packets lost in this interval (UDP mode only) |
+| `cpu: L%/R%` | Local CPU / Remote CPU usage percentage |
+| `!` | Warning: CPU usage exceeds 70% on either side |
 
 ## Complete CLI Reference