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

## 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.