refactor: split test.py into modular pytest integration tests + benchmark tool

- tests/integration/common.py: ServerManager (reuses server across tests),
  run_client, test data generation, verification utilities
- tests/integration/test_tcp.py: 14 TCP transport correctness tests
- tests/integration/test_ssh.py: 10 SSH transport tests (skip when unavailable)
- tests/integration/test_tls.py: 5 TLS encryption tests (new coverage!)
- tests/integration/test_features.py: 13 feature tests (incremental, delete,
  exclude, include, max/min size, bwlimit, dry run, archive, progress)
- tests/integration/test_preflight.py: 7 CLI validation/error tests
- benchmark/bench.py: standalone benchmark with JSON output, p50/p95, multi-run
- Updated Dockerfile with python3-pytest, openssl, openssh-client
- Updated CI to use pytest (gitea.tap-tap.win/taptap/fastsync-ci:v6)
- Removed old monolithic test.py

tests/conftest.py

@@ -0,0 +1,5 @@
+"""Shared pytest configuration for integration tests."""
+import os
+import sys
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "integration"))