fix: invert y-axis label row index to match plot orientation

[meszmate]

Fixes #110.

Summary

• The plot is rendered top→bottom, but TickSet.updatePositions stored tick positions using mapToResolution directly, which puts the smallest value at position 0. mapY (used for drawing the data) flips the coordinate so the largest value lands at screen row 0, leaving the y-axis labels mirrored relative to the plot.
• renderPlotRow now looks up the tick label by plot_height - 1 - row_index, matching the rendered plot orientation (largest value at top, smallest at bottom).
• Drops the _ = plot_height discard now that the parameter is used.

Test plan

• zig build run-charts (or equivalent) and confirm y-axis labels are ordered largest at top, smallest at bottom.
• Charts with show_labels = false still render unchanged.