[Feature]: HTCondor scheduler (shared `_htcondor` core) for BatchBackend

[aldbr]

User Story

As the WMS consumer,
I want BatchBackend(SSHTransport|LocalTransport, HTCondorScheduler) working against a real
schedd,
So that the second scheduler proves the Scheduler protocol generalises, and the _htcondor core
is ready for reuse by the HTCondor-CE backend.

Feature Description

• _htcondor internal module (Tier C, shared with #): submit-description builder
  from SubmissionSpec (incl. queue N for count fan-out, should_transfer_files/
  transfer_output_files from OutputSpec), condor_q/condor_history ClassAd/JSON parsing,
  native state map incl. hold codes/subcodes (port DIRAC's STATES_MAP/HOLD_REASON_SUBCODE).
• HTCondorScheduler: drives a schedd via the transport (CLI condor_submit/condor_q/
  condor_rm over Transport.run); stages_own_files=True — HTCondor declares file transfer in
  the submit description (the cross-scheduler staging axis from IC-ADR-001 §3).
• Phase 2 wiring: contract suite against the htcondor stack's plain-schedd path
  (markers scheduler and htcondor); ssh-condor stack optional here, may land later.

Definition of Done

• Contract suite green against the htcondor stack (plain schedd)
• count > 1 submits as one queue N (asserted)
• Output sandbox via native file transfer, honouring OutputSpec at submit time
• State mapping unit-tested against canned condor_q/condor_history output (port DIRAC's
  test fixtures)
• _htcondor has no scheduler-class dependencies (importable by the CE backend, #)

Alternatives Considered

• Python htcondor bindings instead of CLI-over-transport — bindings can't run over SSH to a
  remote schedd; CLI-over-transport keeps the transport axis orthogonal. Bindings remain an option
  for the monolithic CE backend (issue-15), where the schedd is reached natively.

Related Issues

Additional Context

IC-ADR-001 §3 "Shared batch-system knowledge": scheduler adapter and monolithic CE share the
_htcondor functions, never a subtype.