Purpose
-------
OI-525 - single source of truth for the ``_dbg()`` import used across OCT
modules. The ``oc_diagnostics`` package is an optional runtime dependency;
when unavailable (dev-time tooling, partial installs, tests running outside
a full environment) ``_dbg()`` becomes a no-op so callers never crash.
Responsibilities
----------------
- Expose ``_dbg`` re-bound from ``oc_diagnostics`` when importable.
- Fall back to a no-op stub whose signature is EXACTLY ``oc_diagnostics._dbg``'s
(names, kinds, defaults) - no extra tolerance, so code that only ever ran
against the stub cannot silently depend on laxness the real callable
rejects (the plan-213 E0 lesson).
- Never raise on import.
Diagnostics
-----------
Domain: OCT-CORE
Levels:
L2 - reserved (this module provides the plumbing and emits nothing itself)
L3 - reserved
L4 - reserved
Contracts
---------
- Every OCT module that needs ``_dbg`` must import from here rather than
duplicating the try/except stub.
- Signature must match ``oc_diagnostics._dbg`` so swap is transparent.
Dependencies
------------
- oc_diagnostics (the real _dbg, re-exported when the package is importable)