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/man/btest.1

@@ -1,5 +1,5 @@
 .\" btest-rs manual page
-.\" Generated for btest-rs v0.5.0
+.\" Generated for btest-rs v0.6.0
 .TH BTEST 1 "2026-03-31" "btest-rs 0.5.0" "User Commands"
 .SH NAME
 btest \- MikroTik Bandwidth Test server and client