Name generated signature parameters the way CPython names them (#8725)

* Leave Rust-only name decoration out of a generated signature

A leading underscore marks an argument Rust sees as unused and `r#`
escapes a Rust keyword. Neither describes the parameter a Python caller
passes, and `r#` is not valid Python, so `inspect.signature()` rejects
any signature carrying one.

Reporting the bare name fixes _sre.template, select.epoll.__exit__ and
bytearray.__reduce_ex__ without touching them, and lets a method name a
parameter after a Rust keyword.

A parameter CPython itself names with a leading underscore, such as
compile's `_feature_version`, reaches Python through a FromArgs field,
which this never sees.

Assisted-by: Claude Code:claude-opus-5

* Name builtin type parameters the way CPython names them

A generated __text_signature__ reports the Rust parameter name, so a
name chosen for Rust reaches anyone reading help() or inspect. Rename
the ones CPython describes differently: list.pop's `i` becomes `index`,
list.count's `needle` becomes `value`, __format__'s `spec` becomes
`format_spec`, bytes.strip's `chars` becomes `bytes`, and
object.__reduce_ex__'s `proto` becomes `protocol`.

Left alone are the parameters CPython reports as `object` or `unused`.
Those are synthesised from the METH_* flags rather than named: CPython
reports slice.indices as ($self, object, /) while its own docstring
calls the argument len.

Assisted-by: Claude Code:claude-opus-5

* Name os parameters the way CPython names them

os.read's `n` becomes `length`, os.lseek's `how` becomes `whence`,
os.kill's `sig` becomes `signal`, os.stat's `file` becomes `path`, and
os.putenv's `key` becomes `name`, among others.

os.chmod is left alone. It reports (path, dir_fd, mode, follow_symlinks)
while the call binds mode second and takes follow_symlinks by keyword, so
the order and the kind are wrong rather than the names, which a rename
cannot fix.

os.fstatvfs is left alone too: it shares one Rust function with statvfs,
which CPython names `path`, so the two aliases cannot report different
names.

Assisted-by: Claude Code:claude-opus-5

* Name sys parameters the way CPython names them

sys.excepthook's (exc_type, exc_val, exc_tb) become (exctype, value,
traceback), sys._getframe's `offset` becomes `depth`, sys.intern's `s`
becomes `string`, and sys.settrace's `tracefunc` becomes `function`.

sys._clear_type_descriptors takes the name CPython gives it, which
Rust spells r#type.

Assisted-by: Claude Code:claude-opus-5

* Name vm stdlib parameters the way CPython names them

deque.rotate's `mid` becomes `n`, deque.extend's `iter` becomes
`iterable`, _sre.ascii_tolower's `ch` becomes `character`,
_operator.neg's `pos` becomes `a`, signal.alarm's `time` becomes
`seconds`, and codecs.register_error's `name` becomes `errors`.

gc.collect and charmap_build keep theirs. Their argument is a FromArgs
struct or a PosArgs bundle, so the reported name is the binding rather
than a parameter, and renaming the binding would only disguise that.

Assisted-by: Claude Code:claude-opus-5

* Name stdlib crate parameters the way CPython names them

unicodedata.category's `character` becomes `chr`, math.isqrt's `x`
becomes `n`, cmath.polar's `x` becomes `z`, struct.calcsize's `fmt`
becomes `format`, binascii.crc32's `init` becomes `crc`, zlib.adler32's
`begin_state` becomes `value`, and array.append's `x` becomes `v`.

array.fromunicode's parameter is `ustr`, which cspell has to be told
about.

array.__deepcopy__ keeps `_memo`, which the generator now reports as
`memo`; CPython calls it `unused`, a name synthesised from METH_O rather
than chosen.

Assisted-by: Claude Code:claude-opus-5

* Name BaseExceptionGroup.split's parameter the way CPython names it

CPython calls the argument of split and subgroup `matcher_value`.

Assisted-by: Claude Code:claude-opus-5

* Strip a generated parameter name with unraw and one prefix check

syn already removes r# through IdentExt::unraw, the way from_args.rs
does for a field name. The empty-name guard never ran: a bare `_` is
Pat::Wild, which func_sig has already refused.

Assisted-by: Claude Code:claude-fable-5-1

* Take object and unused from CPython's Clinic signatures

CPython also falls back to these names for METH_O, but list.append and
insert, set.add, discard and remove, _weakref.getweakrefcount and
getweakrefs, sys.displayhook and getrefcount, and array.__deepcopy__
get them from an Argument Clinic signature. The earlier commit that
kept array.__deepcopy__'s `_memo` was wrong to call `unused`
synthesised.

test_pydoc's set.add summary-line tests now pass.

Assisted-by: Claude Code:claude-opus-5

Lib/test/test_pydoc/test_pydoc.py

@@ -1604,12 +1604,10 @@ class TestDescriptions(unittest.TestCase):
         self.assertEqual(self._get_summary_line(''.lower),
             "lower() method of builtins.str instance")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_unbound_builtin_method_o(self):
         self.assertEqual(self._get_summary_line(set.add),
             "add(self, object, /) unbound builtins.set method")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_bound_builtin_method_o(self):
         self.assertEqual(self._get_summary_line(set().add),
             "add(object, /) method of builtins.set instance")