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/integration/test_preflight.py

@@ -0,0 +1,81 @@
+"""CLI validation and preflight checks."""
+import subprocess
+import sys
+import os
+import pytest
+
+sys.path.insert(0, os.path.dirname(__file__))
+from common import BUILD_DIR, CLIENT_CMD, SERVER_CMD
+
+
+class TestHelp:
+    def test_client_help(self):
+        r = subprocess.run(CLIENT_CMD + ["--help"], capture_output=True, text=True)
+        assert r.returncode == 0
+        assert "Usage:" in r.stdout
+        assert "SSH transport" in r.stdout
+
+    def test_server_help(self):
+        r = subprocess.run(SERVER_CMD + ["--help"], capture_output=True, text=True)
+        assert r.returncode == 0
+        assert "Usage:" in r.stdout
+
+
+class TestSSHDetection:
+    def test_remote_dest_detected(self):
+        """Posix-style SSH dest should be detected and fail gracefully."""
+        r = subprocess.run(
+            CLIENT_CMD + ["/x", "somehost:/y"],
+            capture_output=True, text=True, timeout=5,
+        )
+        assert r.returncode != 0
+        stderr = (r.stderr or "").lower()
+        assert "ssh" in stderr or "error" in stderr or "could not" in stderr
+
+    def test_local_dest_not_ssh(self):
+        """Local path should not be detected as SSH."""
+        r = subprocess.run(
+            CLIENT_CMD + ["/tmp/x", "/tmp/y"],
+            capture_output=True, text=True, timeout=5,
+        )
+        # Should fail with connection error (no server), not SSH error
+        assert r.returncode != 0
+
+
+class TestServerStdio:
+    def test_stdio_mode_starts(self):
+        """Server --stdio should start and wait for stdin."""
+        try:
+            r = subprocess.run(
+                SERVER_CMD + ["--stdio"],
+                capture_output=True, text=True, timeout=3,
+            )
+            # Should exit with error (no data on stdin) or timeout
+        except subprocess.TimeoutExpired:
+            pass  # Expected: server waiting for stdin
+
+
+class TestServerPort:
+    def test_invalid_port(self):
+        """Server should reject invalid port numbers."""
+        r = subprocess.run(
+            SERVER_CMD + ["-p", "99999"],
+            capture_output=True, text=True, timeout=5,
+        )
+        assert r.returncode != 0
+
+    def test_default_port(self):
+        """Server should start on default port 8080."""
+        proc = subprocess.Popen(
+            SERVER_CMD, stdout=subprocess.DEVNULL, stderr=None,
+        )
+        try:
+            import socket, time
+            time.sleep(0.5)
+            with socket.create_connection(("127.0.0.1", 8080), timeout=2):
+                pass  # Port is listening
+        except (ConnectionRefusedError, OSError):
+            pytest.fail("Server not listening on default port 8080")
+        finally:
+            proc.terminate()
+            proc.wait(timeout=5)