H1: Delete 7 dead _pre_validate_* static methods from SchemaValidator
— exact duplicates of plugin pre_validate() methods, never called
after _pre_validate_chart_type() was updated to delegate to plugin.
H2: Inline DatasetValidator._normalize_xy_config/_normalize_table_config
into XYChartPlugin/TableChartPlugin.normalize_column_refs() and delete
both DatasetValidator helper methods. The 5 other plugins already
called _get_canonical_column_name directly; XY and Table now match.
H3: Add generate_name()/resolve_viz_type() to ChartTypePlugin protocol
and BaseChartPlugin, implement in all 7 plugins. Replace the 7-arm
isinstance chain in generate_chart_name() and the 7-arm elif chain
in _resolve_viz_type() with single-line registry dispatch.
H4: Add a sync comment above _CHART_TYPE_ERROR_HINTS to document that
it must stay in sync with the plugin registry.
M4: Move logger=logging.getLogger(__name__) from inside
XYChartPlugin.get_runtime_warnings() to module scope.
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