enhance: render Others slice with secondary foreground color

[JC-Chung]

Changes

• Render the Others slice with secondaryForeground (Brush.FG2) instead of the next palette color. When the number of authors above 1% is a multiple of 10, the palette index wraps around to 0, making Others the same Orange as the adjacent top-author slice — the two blend together with no visible boundary.
• Draw a 1px separator line at the top of the pie (the head-to-tail seam) via a new BorderBrush property bound to Brush.Window. Without it, when all authors are above 1% and the count is 11/21/31..., the last author's slice is also Orange and adjacent to the top author's slice, which the Others fix cannot cover.
• Move var brush = s_brushes[brushIndex] into the percent > 0.01 branch.
• Rename _lastHoverred to _lastHovered (no behavior change).

[love-linger]

I've pushed my implementation for this feature. See dbfde1a

[JC-Chung]

@love-linger
Separately, I've also been working on the label overlap issue — see e0b6eb5 (on my fix/statistics-pie-chart-overlap branch).
Labels of small adjacent slices share nearly the same Y position and stack on top of each other, so they're now sorted per side and pushed apart to keep a minimum vertical gap, connected by elbow-style leader lines.

One trade-off: when more labels on one side than can fit vertically, labels for the smallest slices are dropped. The slices themselves are unchanged and still show details on hover via the tooltip.

Before / after: