ci: parallelize integration suite with pytest-xdist; fast PR gate + full coverage on merge

- Add pytest-xdist to the CI Dockerfile (image -> v10).
- Worker-isolate TEST_DATA_DIR (PYTEST_XDIST_WORKER) so concurrent xdist
  workers never collide on shared-filesystem fixtures.
- Register a 'ci' marker and tag a fast representative subset of integration
  tests (basic TCP, incremental, compression, delete, basis, append).
- ci.yaml: lint + build + unit + the marked subset (-n4) on every PR; the
  full integration suite plus sanitizer/fuzz/coverage/valgrind run only on
  push to dev/main.
- Integration step runtime drops from ~860s (serial) to ~230s (-n4); the PR
  gate lands well under ~3 minutes.

tests/integration/test_tcp.py

@@ -49,10 +49,12 @@ def _run_tcp_test(name, port, flags, use_metadata=True, posix=False):
 
 
 class TestTCPStandard:
+    @pytest.mark.ci
     def test_standard(self, shared_server):
         r = _run_tcp_test("Standard", shared_server.port, [])
         assert r["status"] == "Success", r["error"]
 
+    @pytest.mark.ci
     def test_posix_args(self, shared_server):
         r = _run_tcp_test("Posix Args", shared_server.port, [], posix=True)
         assert r["status"] == "Success", r["error"]
@@ -63,10 +65,12 @@ class TestTCPStandard:
 
 
 class TestTCPFlags:
+    @pytest.mark.ci
     def test_multithreading(self, shared_server):
         r = _run_tcp_test("Multithreading (-m)", shared_server.port, ["-m"])
         assert r["status"] == "Success", r["error"]
 
+    @pytest.mark.ci
     def test_compression(self, shared_server):
         r = _run_tcp_test("Compression (-c)", shared_server.port, ["-c"])
         assert r["status"] == "Success", r["error"]