fix: render dict **-unpacking as **value instead of None: value

[gaoflow]

• I did not use AI
• I used AI and thoroughly reviewed every code/docs change

Description

ExprDict.iterate renders the None key that the AST uses to mark a **-unpacked value as a literal None: value, producing invalid (and semantically wrong) Python:

def f(a={**base, "x": 1}): ...   # default renders as  {None: base, 'x': 1}
def g(b={**d1, **d2}): ...       # default renders as  {None: d1, None: d2}

_build_dict faithfully preserves the AST's None-key unpacking marker, but iterate then renders it with the literal text "None". This affects any rendered signature/default/annotation containing a dict with **-unpacking.

The fix renders entries whose key is the unpacking marker as **value (no key: part):

def f(a={**base, "x": 1}): ...   # now renders as  {**base, 'x': 1}
def g(b={**d1, **d2}): ...       # now renders as  {**d1, **d2}

A genuine None literal key is an ast.Constant, not the AST's unpacking marker, so it is left untouched ({None: 1, "y": 2} still renders as {None: 1, 'y': 2}).

Verification

• Added test_render_dict_with_unpacking covering **-spread, multiple spreads, and the literal-None-key case.
• Round-trip oracle: every rendered form now re-parses AST-equal to its source.
• Full griffelib suite passes with no new failures (the unrelated test_finder.py::test_pth_file_handling* and test_api/test_git failures are pre-existing on this environment, identical with the change stashed).

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This pull request was prepared with the assistance of AI, under my direction and review.

[pawamoy]

Thanks a lot @gaoflow! It all looks good 🙂