- 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.
