Render crosshair lines under chart content

[Federicocervelli]

Summary

• split crosshair DOM into line and overlay layers
• keep line layer behind the WebGL canvas by default
• keep labels and markers above chart content
• clear the WebGL canvas transparently so chart background comes from the root layer

Tests

• bun run typecheck
• bun run test:visual
• bun run pages:build

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep inside axis labels above the canvas

When an axis is configured with position: "inside", AxisOverlay appends its tick-label divs directly to chart.plotElement without assigning a z-index, so adding zIndex = "1" to the canvas puts the WebGL canvas above those labels in the plot stacking order. In that configuration the inside tick labels are now painted underneath grid/series pixels instead of overlaying the chart as before; give the inside-axis labels a z-index above the canvas or avoid raising the canvas above existing plot overlays.

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