The reader stored a series-level `c:dLbls/c:showVal` in
`Series.data_labels`, but every drawing path tested only the chart-wide
`Chart.data_labels`: a series that asked for labels got none, and a
series that asked for none got them. The series' own setting now wins,
and a bare `<c:showVal/>` (no `val` attribute) means true, as the
schema's default.
The plot group's own setting is respected too: a group that asks for no
labels keeps them off, even when another group of a combo chart asks for
them. Before, any group that asked turned the chart-wide setting on, and
the group that said no inherited it.
Tests: `crates/pptx/tests/chart_data_labels.rs` - a bare series
`showVal` shows that series' labels while its siblings stay quiet; an
explicit `val="0"` on the first series keeps it quiet under a chart-wide
on; a combo chart keeps each group's own setting; all three settings
round-trip. Red-proof: with the series-level setting ignored, the first
two fail; with the group's setting ignored, the combo chart's bar group
shows labels it asked to keep quiet.
The model gains `Chart::series_data_labels`, the one place that decides
whether a series' labels show; the render paths (bar, column, line,
area, pie, doughnut, radar, scatter) call it.
aiongg