f5#899

Ports the applicable parts of f5#896 to GroupedBar:

Fixes lingering groups on rapid data updates. GroupedBar's group exit transition had no interrupt handler: when a re-render arrived mid exit fade, the group was never removed — its class had already been swapped to barGroupExit, so no later data join could re-adopt or clean it up, leaving a semi-transparent ghost group (with its bars frozen mid-squash) in the DOM forever. Fixed by removing the node on interrupt, mirroring StackedBar's group-level handling.

New barStyle accessor. Same feature as StackedBar's in f5#896: (datum, indexWithinGroup) => StyleDeclaration applies custom inline styles per bar. Keys the component manages itself (fill, cursor, mask) are merged into the render transitions as target values, so they win over defaults like the color accessor while staying animated; other keys are applied instantly and clean up when no longer returned. Unlike StackedBar, opacity is not in the managed set — GroupedBar never animates bar-level opacity.

Depends on f5#896 — the two Core commits (applyInlineStyles, StyleDeclaration export) are cherry-picked from it and will dedupe once it merges.

• Dev examples (Per-Bar Styles: actual-vs-projected chart with a movable boundary demonstrating barStyle application and cleanup)
• Wrappers (Angular regenerated via autogen; others regenerate to no diff)
• Docs (new "Custom Bar Styles" section with a live example; barStyle appears in the auto-generated props table)

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