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_append.py

@@ -10,6 +10,8 @@ import os
 import random
 import shutil
 
+import pytest
+
 from common import (
     TEST_DATA_DIR,
     run_client, CountingProxy, clean_dir,
@@ -46,6 +48,7 @@ class TestAppend:
     def _dest_file(self, source, dest, rel):
         return os.path.join(get_dest_received_dir(dest, source), rel)
 
+    @pytest.mark.ci
     def test_append_resumes_short_dest_atomically(self, shared_server):
         """A shorter dest with a MATCHING prefix is resumed; the reconstructed
         file is byte-identical to the source."""