[Feature]: Scheduler protocol + Slurm scheduler + generic BatchBackend

[aldbr]

User Story

As the WMS consumer,
I want BatchBackend(SSHTransport, Slurm) and BatchBackend(LocalTransport, Slurm) working
end-to-end against the containerised Slurm stacks,
So that the N+M composition promise of IC-ADR-001 §3 is proven on the first real scheduler.

Feature Description

• Scheduler protocol finalised: submit_cmd(spec), parse_status(raw), kill_cmd(ids),
  stages_own_files flag (+ stage_inputs/collect_outputs hooks where needed). Evaluate the
  ADR's open question — promote staging to a Stager collaborator — and record the outcome.
• _slurm internal module (Tier C): sbatch script/option generation from SubmissionSpec
  (incl. count→--array for identical fan-out), squeue/sacct parsing, native-state →
  JobStatus map (port DIRAC's mapping: PENDING/SUSPENDED/CONFIGURING→waiting, COMPLETED→done,
  CANCELLED/PREEMPTED→aborted…).
• Slurm scheduler consuming _slurm; registered via intercede.schedulers.
• Generic BatchBackend (Tier B): satisfies JobBackend + OutputRetriever
  (destructive=False) + Cancellable + Purgeable + LoadReporter; sandbox staging via
  Transport.put/get when stages_own_files is false; workdir layout per job; JobHandle
  carries host/workdir routing (replaces DIRAC's ssh<batch>:// ref encoding).
• Phase 2 wiring: the integration contract suite (submit → status → fetch → kill, fetch-twice,
  partial-failure maps) runs against ssh-slurm and local-slurm stacks via markers.

Definition of Done

• Contract suite green on ssh-slurm and local-slurm stacks in CI
• count > 1 submits via a single --array (asserted, not N submits)
• fetch_output streams an arbitrary output sandbox (not just stdout/stderr) with bounded
  materialisation (size/count/timeout/path-containment)
• LoadReporter.counts() from squeue; unknown ids → JobStatus.UNKNOWN

Alternatives Considered

• Per-combination classes (SSHSlurmBackend, LocalSlurmBackend…) — N×M explosion; rejected.
• Porting DIRAC's remote-executed SLURM.py driver — requires remote python; dropped (see
  issue-10).

Additional Context

This issue is the template for every further scheduler.