Fix composition rendering broken by plotnine 0.15.x API changes

[Copilot]

The notebooks CI job fails because plotnine_extra's composition code calls two APIs removed/changed in plotnine 0.15.x: p9GridSpec.from_layout() (class method no longer exists) and theme._setup() (renamed to theme.setup(plot)).

Changes

• composition/_compose.py — _generate_gridspecs: Replace p9GridSpec.from_layout(self.layout, ...) with a direct constructor call using the layout's nrow, ncol, widths, and heights:

# Before (broken in 0.15.x):
self._sub_gridspec = p9GridSpec.from_layout(
    self.layout, figure=figure, nest_into=container_gs[0]
)

# After:
self._sub_gridspec = p9GridSpec(
    self.layout.nrow,
    self.layout.ncol,
    figure,
    width_ratios=self.layout.widths,
    height_ratios=self.layout.heights,
    nest_into=container_gs[0],
)

• composition/_compose.py — draw: Replace theme._setup(figure, None, title, subtitle) with the current theme.setup(plot) API, using a SimpleNamespace mock that satisfies the interface (figure, axs, labels):

from types import SimpleNamespace
_mock_plot = SimpleNamespace(
    figure=self.figure,
    axs=[],
    labels={
        "title": self.annotation.title or "",
        "subtitle": self.annotation.subtitle or "",
    },
)
self.theme.setup(_mock_plot)