Purpose
-------
OI-525 regression tests — ``oct.core.diagnostics`` must provide a single
``_dbg`` entry point that re-binds ``oc_diagnostics._dbg`` when available
and falls back to a silent no-op stub otherwise. This eliminates the
duplicated per-module try/except that previously existed in every
``oct/git/*.py`` file.
Responsibilities
----------------
- Importing ``_dbg`` from ``oct.core.diagnostics`` never raises.
- The resolved ``_dbg`` is callable with ``(domain, func, msg, level)``
and arbitrary kwargs, and returns ``None``.
- When ``oc_diagnostics`` is importable, ``_dbg`` is the real callable
(``oc_diagnostics._dbg``) or a trivial bind of it — not a no-op shim.
- When ``oc_diagnostics`` is unavailable, a re-evaluated module exposes
a no-op fallback.
Diagnostics
-----------
Domain: CORE-TESTS
Levels:
L2 — test lifecycle
L3 — assertion details
L4 — deep tracing
Contracts
---------
- Signature must match ``oc_diagnostics._dbg`` so callers can switch
transparently.