CCE: compute boundary Du<Dy<J>> from buffered R and Dr<J>

[gda98re]

Adds Tags::Du<Tags::Dy<Tags::BondiJ>> to
characteristic_worldtube_boundary_tags (needed by CauchySecondOrder) and computes its boundary value.

The boundary value of Tags::Du<Tags::Dy<Tags::BondiJ>> is computed directly by BondiWorldtubeDataManager from quantities already in the buffer:

• At every buffered time inside the current interpolation sub-window, extract the modes of R and Dr<BondiJ> from coefficients_buffers_, convert them from Goldberg to libsharp layout, and inverse-SWSH-transform to nodal values at the boundary.
• Form the nodal product dy_j = 0.5 * R * Dr<BondiJ> at each buffered time.
• At every angular collocation point, call SpanInterpolator::derivative on the resulting time series to obtain the time derivative at the requested time, which is written into BoundaryValue<Du<Dy<BondiJ>>>.

create_bondi_boundary_data does not populate Du<Dy<BondiJ>>, so the boundary-comparison loops that build their "expected" Variables from it exclude that tag via tmpl::list_difference (with a comment explaining why): Test_WorldtubeData, Test_AnalyticBoundaryDataManager, and the H5 / Gh / Analytic *BoundaryCommunication tests.

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