Enable parallel, resumable experiment runs

[alpaylan]

Follow-up to #57/#58. Makes it possible to run many experiment dispatches (e.g. the 21 ifc-proplang-rocq-* variants) in parallel and have every result persist — even when a job is killed at the timeout.

Changes

1. Per-test concurrency group. group: run-experiment-${{ github.ref }} + cancel-in-progress meant dispatching any test cancelled an in-flight run for a different test on the same branch (only the latest survived). Fold inputs.tests into the group key so distinct tests run concurrently; re-dispatching the same test still supersedes its own prior run.

2. Rebase-retry on the result push. Parallel jobs commit disjoint store files from the same base commit, so a bare git push is rejected non-fast-forward for all but the first (a ref race, not a content conflict). Because the files are disjoint, rebasing onto the updated branch never conflicts — fetch + rebase + retry lets every job land its results.

3. Commit partial results on kill. The commit step was success()-only, so a job killed at the timeout never persisted progress (only the always-uploaded artifact). Now:

   • if: ${{ inputs.commit-results && always() }} so it also runs on failure/cancel/timeout,
   • guards on the store file existing, tags interrupted commits [partial: <status>],
   • job timeout-minutes 360 → 350 so the job self-cancels below GitHub's hard 6 h kill, leaving headroom for the commit to push.
     store.push appends + flushes per trial, so the committed partial store lets a re-run resume via the incremental per-trial dedup rather than restarting.

Validation

Local two-job, disjoint-file push race (same base commit):

1) bare push:   jobA OK ; jobB REJECTED (non-fast-forward)   <- the bug
2) rebase-retry: jobB push after rebase: OK
final remote:   store-A.jsonl=result-from-jobA, store-B.jsonl=result-from-jobB  (clean history, no conflict)

Confirmed in source that store.push (store.rs) opens append + flush()es each metric and the driver calls it per trial, so a killed job's committed store holds every completed trial. The always() + timeout-headroom behavior relies on GitHub running always() steps within the cancellation grace below the hard 6 h limit.

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