[Feature]: Implement an ARC 7 CE - Slurm integration test in a CI workflow

## User Story

As a contributor for the ARC module,
I want an ARC 7 CE wired to Slurm running in CI with a real submit → monitor → retrieve test,
So that changes to the ARC path are validated end-to-end against a real LRMS and no regressions slip in.

## Feature Description

Stand up the **`arc-slurm` integration-test stack** (IC-ADR-002): an ARC 7 Compute Element
(REST/AREX) wired to a single-node Slurm LRMS, plus CI that builds the image, boots it, and runs a
full **submit → monitor → retrieve** test with the real ARC client tools
(`arcsub`/`arcstat`/`arcget`). ARC's `arcctl test-ca` mints CA + host cert at install → no grid-PKI
in CI; the client credential is written to the shared `credentials/` volume.

Harmonize onto the scaffold from # per the checklist.

## Definition of Done

- [ ] Image builds & boots in CI; healthcheck (`curl -k` the REST info URL) passes
- [ ] `arcsub` submits, `arcstat` reaches `Finished`, `arcget` retrieves, output asserted
- [ ] ARC/Slurm logs uploaded as artifacts on every run
- [ ] Harmonized onto the scaffold

## Alternatives Considered

- **Official ARC image** — none is maintained; pinned RPMs on EL9 is the supported path (ADR §Facts).

## Related Issues

## Additional Context