feat: add renderHalfHour prop for :30 time-column labels

[anton-patrushev]

Summary

• New renderHalfHour prop on CalendarBody plumbed through BodyContext.
• TimeColumn renders an optional :30 label between each hour, positioned at the midpoint and counter-scaled to keep text readable while pinch-zooming.

Ported from core-mobile patch @howljs+calendar-kit+2.5.6+022+half-hour-labels.

Stacked on top of #20.

Test plan

• Render a calendar passing renderHalfHour; verify :30 labels appear between hours.
• Omit renderHalfHour; verify no extra labels render.
• Pinch-zoom; verify half-hour labels stay legible (counter-scaled).