The live mutable registry of lazy imports is exposed (undocumented) as
``sys._lazy_modules``. ``sys.lazy_modules`` remains an immutable
``frozendict`` snapshot rebuilt on each access via ``sys.__getattr__``.

Tests reset state with ``sys._lazy_modules.clear()``; the temporary
``_imp._clear_lazy_modules()`` helper is removed.

- Reword NEWS to describe contract (read-only frozendict mapping module
  names to frozensets of submodule names) rather than implementation
  details. Match PEP 810 wording ("mapping").
- Drop the sys.__getattr__ docstring and restore the borrowed-reference
  comment in _PySys_Create.
- Use "mapping" in test docstrings/comments that describe the contract
  while leaving "snapshot" in implementation-detail spots.

Drop test_lazy_modules_is_immutable; the frozendict assertion already
covers immutability. Trim the trailing sentence in the NEWS entry that
restated the same contract.

Drop the defensive PyUnicode_Check that raised TypeError, and use the
public PyUnicode_EqualToUTF8 instead of _PyUnicode_EqualToASCIIString.
A non-string name now falls through to AttributeError, matching the
existing C-module __getattr__ idiom in _ctypes/callproc.c and
zlibmodule.c.

Switch sys.__getattr__ to METH_VARARGS with PyArg_UnpackTuple, matching
the existing C-module __getattr__ pattern in _ctypes/callproc.c and
zlibmodule.c.

…pshot

Pre-size the temporary dict in _PyImport_GetLazyModulesSnapshot via
_PyDict_NewPresized to avoid resizes during the inline copy. Add a fast
path that returns an empty frozendict directly when the registry has no
entries, skipping the temporary dict allocation entirely.

Drop _PyDict_NewPresized and the empty-registry short-circuit. Both
were micro-optimizations with negligible impact for typical registry
sizes; the simpler unconditional PyDict_New + iterate path is easier
to read.

- Add ``sys.lazy_modules`` entry to Doc/library/sys.rst (frozendict
  snapshot, fresh per access, versionadded 3.15)
- Fix test_builtin.test_vars: sys.lazy_modules is served by
  sys.__getattr__ so it appears in dir() but not vars()
- Fix test_inspect.test_sys_module_has_signatures: sys.__dir__ and
  sys.__getattr__ are METH_NOARGS/METH_VARARGS builtins without
  Argument Clinic text signatures; add them to no_signature

…s-ref from docs