Fix ephemeral scheduler pytest fixture cleanup

[8Dionysus]

Problem

Ephemeral scheduler trials put pytest's tmp_path and tmpdir fixtures under the shared host temporary root because the child pytest commands did not set --basetemp. The outer TemporaryDirectory then removed only the job artifact root, leaving fixture generations behind in pytest-of-*.

Change

Bind each scheduler pytest process to a private basetemp under its job artifact root:

• static collection: pytest-basetemp/collection
• each static shard: pytest-basetemp/shard-N
• ordinary serial or xdist execution: pytest-basetemp/pytest

xdist continues to create popen-gwN worker directories beneath the per-process root. Explicit artifact and receipt runs retain the artifact tree; ephemeral runs reclaim it through the existing TemporaryDirectory owner.

Validation

• PYTHONDONTWRITEBYTECODE=1 python3 -m pytest -q -p no:cacheprovider tests/test_validation_scheduler_experiment.py (18 passed)
• ruff check scripts/pytest_scheduler_experiment.py tests/test_validation_scheduler_experiment.py
• python3 -m py_compile scripts/pytest_scheduler_experiment.py tests/test_validation_scheduler_experiment.py
• git diff --check

No full scheduler corpus run was needed for this focused lifecycle change.