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/common.py

@@ -13,7 +13,12 @@ PROJECT_ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "..
 BUILD_DIR = os.path.join(PROJECT_ROOT, "build")
 SERVER_CMD = [os.path.join(BUILD_DIR, "server")]
 CLIENT_CMD = [os.path.join(BUILD_DIR, "client")]
-TEST_DATA_DIR = os.path.join(PROJECT_ROOT, "test_data")
+# Under pytest-xdist each worker process gets its own PYTEST_XDIST_WORKER id
+# ('gw0', 'gw1', ...).  Worker-key the transient working dir so concurrent
+# workers on the shared filesystem never collide on fixtures.  Outside xdist
+# (or with -n1) this stays the historical 'test_data' path.
+_WORKER = os.environ.get("PYTEST_XDIST_WORKER")
+TEST_DATA_DIR = os.path.join(PROJECT_ROOT, f"test_data-{_WORKER}" if _WORKER else "test_data")
 
 
 class ServerManager: