[Feature]: ARCBackend — submit + status over ARC 7 REST, token auth

先阅读 DIRAC 的 AREXComputingElement.py 和现有的 contract interfaces，然后检查 arc-slurm stack 及其 manifest。该 issue 除了 config/proxy/ 和 remote arc marker 外，没有指出实现文件或测试文件，因此请在编码前梳理这些集成点。列出的 token/proxy contract、bulk、state、retrieval、delegation 和 registration 检查全部通过后，即表示完成。

User Story

As the WMS consumer,
I want an ARCBackend that submits and monitors jobs on an ARC 7 CE via REST with token auth,
So that the primary grid-CE path (DIRAC's AREX CE) exists on the new contract, validated against
the arc-slurm stack.

Feature Description

Essential half of the AREX port (IC-ADR-001 migration map):

• ARCBackend: implements Submitter + Monitor against the ARC REST
  API (/arex/rest/1.0) using pyarcrest (evaluate fitness; fall back to httpx against the REST
  spec if it can't do async/bulk cleanly — record the choice).
• Job description generation from SubmissionSpec (XRSL or ADL — pick one, document why;
  DIRAC builds XRSL), incl. input-sandbox declaration and queue targeting.
• Bulk semantics: ARC REST supports multi-job operations — submit count copies and poll many
  ids per call; per-id failures ride the outcome maps.
• State normalisation: ARC state map → JobStatus (port DIRAC's STATES_MAP, incl. the
  auto-kill-on-Hold question — surface as policy, don't hide it in the backend).
• Token auth per IC-ADR-003: bearer header, audience exposure. Proxy/delegation
  is issue-14.
• OutputRetriever (destructive=False): list the ARC session dir, stream every member into
  dest file-by-file, return the JobOutput manifest (.stdout/.stderr/.files/.log).
  Bounded materialisation enforced inline: byte ceiling mid-stream, file-count ceiling,
  per-transfer timeout, path containment (../absolute members rejected) — IC-ADR-001 §4.
• Cancellable.kill / Purgeable.purge (DIRAC killJob/cleanJob): bulk, per-id outcomes.
• LoadReporter.counts() from the CE info endpoint.
• SupportsLiveDiagnostics.diagnostics(): the diagnose/errors endpoint — the one genuinely
  independent, non-destructive pre-completion fetch.
• Proxy/delegation auth: create delegation (CSR → sign with the
  proxy chain → upload), renew, AlwaysIncludeProxy parity where tokens+proxy coexist. Add the
  config/proxy/ configuration axis to the arc-slurm stack (ADR-002 §7) to test it.
• Fetch-twice regression test proving re-fetchability (destructive=False) — the counterpart
  of HTCondor's one-shot test.
• Phase 2 wiring: contract suite (submit/status subset) against the arc-slurm stack
  (markers remote and arc).

Definition of Done

• Full contract suite (all capabilities) green against arc-slurm, both token and proxy
  configs
• count > 1 submission bundles one Submission with per-copy failure reporting
• Unknown/expired ids → JobStatus.UNKNOWN (never dropped)
• State map unit-tested; Hold handling decided and documented
• Fetch-twice test green; containment/ceiling tests green (hostile filenames, oversized
  output)
• Delegation renewal covered by a test (short-lived test-CA proxy)
• arc-slurm stack gains config/proxy/; manifest updated
• Registered via intercede.backends as arc

Alternatives Considered

• Wrapping the arc CLI tools — spawns processes, brittle parsing; REST is first-class in ARC 7
  and DIRAC's AREX already proves it.

Related Issues

Additional Context

ARC floor is 7.x (IC-ADR-002 §8). DIRAC reference: `AREXComputingElement.py