Build plano-convex bar shapes as single coherent paths (fix internal seam when stroked)

[postfixNotation]

Summary

Builds the plano-convex bar shapes as a single continuous Path instead of several independent subpaths. Filling already rendered correctly, but stroking the outline (via DefaultBar's border) drew the rectangle's interior edge as a stray line across each bar at the body/cap junction. Merging body and cap into one contour removes that internal seam.

Background / root cause

DefaultVerticalPlanoConvexShape, DefaultHorizontalPlanoConvexShape and the index != 0 branches of VerticalPlanoConvexShape / HorizontalPlanoConvexShape assembled their outline from separate subpaths (addRect + addArc, or two addPath unions).

A single Path containing abutting subpaths fills as the correct union in one pass, so the fill output was fine. But Modifier.border(border, shape) strokes every contour of the outline, including the rectangle's top/side edge that sits interior to the shape. The result is a visible line where the body meets the convex cap — clearly seen on stacked bars, where it appears at the junction of every segment.

Changes

• Merge body and cap into one contour via union (+) in DefaultVerticalPlanoConvexShape, DefaultHorizontalPlanoConvexShape, and the index != 0 branches of VerticalPlanoConvexShape / HorizontalPlanoConvexShape. The union removes the internal edge while leaving the filled area unchanged.
• Leave the bi-convex shapes unchanged: their closing difference (-) already normalizes the boundary into a clean single contour.
• index == 0 branches: return the (optionally inverted) path explicitly instead of mutating the wrapped outline in place — a small readability cleanup adjacent to the same code.

Result

• One coherent contour per shape — no internal subpath seam.
• Correct rendering for both fill and stroke (border).
• Fill area and geometry are unchanged; the fix only affects the stroke/border case.

Screenshots

Stacked bars rendered with a border (the case that exposed the seam):

Before	After

In Before, a horizontal line cuts across each segment where the body meets the cap. In After, only the outer outline is stroked.

Testing

Visual checks via Compose @Preview for vertical and horizontal plano-convex bars, single and stacked, both filled and with a border. Confirmed the fill output is unchanged and the internal seam is gone under stroking.

Notes for reviewers

• No new imports: the + (union) and - (difference) operators wrap Path.op(..., PathOperation.…).
• Allocations are in the same ballpark as before (index != 0 performs two differences + one union per createOutline, vs. two differences + two addPath previously); createOutline is not called per frame.