The `test` job in ci.yml becomes a 4-way `test-shard` matrix (pytest-split,
balanced by a committed .test_durations), aggregated into a single `test`
status check so branch protection is unchanged.
release.yml's duplicate full-suite `test` job is removed — it re-ran the
same ~10 min suite a second time on every push to main/feature branches.
release.yml is now image-build only; the advisory ruff/mypy steps move to
a lean `lint` job in ci.yml.
Net: ~10 min -> ~3 min wall-clock per push, and the suite runs once
instead of twice.
