ci(experimental): skip or stop superseded runs instead of cancelling them, so commits are not marked red #1938

Since #1905, experimental.yml cancels a superseded run (cancel-in-progress on main pushes and PRs). GitHub counts cancelled as a failure in the commit rollup. Measured on 604998428, 6b16545d2, 9bc509686 and c716b64b6: each has statusCheckRollup.state FAILURE while its only non-success checks are cancelled Experimental jobs. Every superseded commit on main therefore shows a red X.

A merged PR's last run is never superseded, so it keeps its runner for the full GraalPy duration. On 2026-10-19, four such runs (#1911, #1932, #1934, #1935) held runners while main's Unit Tests sat queued.

Plan, combining the options discussed with the maintainer:

• Drop cancel-in-progress, so a run is never cancelled.
• Add a gate job that skips the whole run when the commit is no longer the head of main, or of its PR, or when the PR is closed. A skipped run does not count against the commit.
• Have the long test steps poll the head about once a minute. When superseded or closed, stop pytest and end the step successfully with a superseded notice and a job-summary line, so the commit stays green and the runner is freed.
• Keep the timeout-before-cap and memory and disk containment from fix(conda): the Python 3.10 conda build fails since soupsieve 3 was pinned #1633/ci: a failed KTLO leg turns main and pull requests red; report it as a warning and exit 0 #1672.
• Update docs/source/contributing/workflows.rst, and pin the behaviour in tests/project.