Fix Figure.clear leaving stale state and decouple ui kwarg routing

[cvanelteren]

Matplotlib's clear() detaches every axes and artist and sets _suptitle to None, but ultraplot kept its own bookkeeping pointing at the destroyed objects: a cleared figure went on handing out dead axes through subplotgrid, _get_subplot, _iter_subplots and _iter_axes, kept a stale gridspec and label counter, leaked figure-level panels, and raised AttributeError from the next format(suptitle=...) because the suptitle artist was gone, so this adds SubplotManager.reset() and overrides Figure.clear() to call it, empty the panel dict, reset the layout flags, and rebuild the label artists via the extracted _init_super_labels() (which also covers clf(), matplotlib's alias for clear()). It further removes a footgun in ui.py, which split figure keywords from subplot keywords by introspecting the signatures of Figure._parse_proj and Figure._add_subplots even though those are pure pass-throughs whose only remaining job was to mirror SubplotManager's parameter list -- a contract nothing enforced, and one that already fired once when collapsing _parse_proj to (*args, **kwargs) made _pop_params see no projection parameters and silently routed proj to the figure, raising from Figure.set(); _pop_params now introspects SubplotManager.parse_proj and .add_subplots, which actually own those parameters, so the delegators are free to collapse back to (*args, **kwargs).

Closes #755, closes #756

[codecov]

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