Propagate parent axes through FilteredTimeSeries constructor

[Copilot]

FilteredTimeSeries.__init__ called super().__init__() with no arguments, causing AxisHandler to be initialized with empty axes — silently dropping any axis naming/transposition metadata from the parent TimeSeries. This made filtered.axes always return () and broke axis-based transposition on filtered series.

Change

• Pass axes=parent_ts.axes to super().__init__() in FilteredTimeSeries, so the filtered series inherits the parent's axis configuration.

# Before: filtered.axes == () regardless of parent
ts = MyTimeSeries(data, axes=('subaperture', 'wavelength'))
filtered = ts.filter(modes=[1, 2])
filtered.axes  # () — axis metadata lost

# After: axes are preserved
filtered.axes  # ('subaperture', 'wavelength')
filtered.get_data(axes=['wavelength', 'subaperture'])  # transposition works correctly

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