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

## 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://` 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.