Allow bar chart data labels to be positioned outside the bars (#500)

[timmolter]

Fixes #500.

Problem

Before 3.6.4, bar chart value labels could be drawn outside (on top of) the bars. That ability was lost when the configurable labelsPosition was introduced (#402) and clamped to [0, 1], which only allows placement inside the bar (baseline → top). Users asked for the "labels on top" behavior back.

Changes

• CategoryStyler / HorizontalBarStyler: relax the labelsPosition upper cap so values greater than 1 are allowed (still rejects < 0); javadoc documents the new semantics.
• PlotContent_Category / PlotContent_HorizontalBar: for labelsPosition > 1, draw the label outside the bar at a fixed pixel gap that does not scale with the bar's size — gap = (labelsPosition - 1) * 100 pixels (e.g. 1.1 → 10px). The inside [0, 1] behavior (a fraction of the bar) is unchanged. Earlier the outside distance scaled with bar height, so taller bars pushed their labels much farther out — fixed here.
• Automatic label font color: when a label is drawn outside the bar it sits on the plot background, so the automatic contrast color is now computed against the plot background instead of the bar fill. This keeps outside labels legible without setting a color manually, and also fixes the stack-sum total labels (which are always drawn outside).
• AxisPair: automatically reserve axis headroom when labels are drawn outside, so they are not clipped at the plot edge (no need to set manual axis min/max).

Demos

• Enabled outside labels in BarChart04 and HorizontalBarChart01.
• Added TestForIssue500 and TestForIssue500_HorizontalBar standalone issue charts (positive and negative values).

Testing

• All 93 existing unit tests pass.
• Verified rendering for vertical and horizontal bars, positive and negative values, and varying bar heights (the outside gap is now constant regardless of bar size, and labels are no longer clipped).

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