Add world axis lines with brighter styling for origin grid lines

[Copilot]

This PR adds special styling for grid lines that pass through the origin (world axis lines) to make them stand out as reference lines, bringing ImPlot3D's visual behavior in line with ImPlot.

Problem

ImPlot3D currently only distinguishes between minor and major grid lines (with 0.3 and 0.6 alpha respectively), but lacks the special highlighting for lines passing through the origin that ImPlot provides. This makes it harder to visually identify the coordinate system's origin in 3D plots.

Before (ImPlot3D):

Reference (ImPlot):

Solution

The implementation adds a third visual tier for grid lines:

• Minor grid lines: 30% opacity (dim, subtle background grid)
• Major grid lines: 60% opacity (medium, regular tick marks)
• World axis lines: 90% opacity (bright, origin reference) ← NEW

Implementation Details

• Detects origin lines by checking if tick position is within 1e-8f of zero (consistent with other coordinate comparisons in the codebase)
• Applies to all three axes (X, Y, Z) and is visible on all plot faces
• Minimal code changes: only 3 lines added + 2 lines modified in the RenderGrid function
• Maintains backward compatibility - existing plots will just get the improved visual hierarchy

Testing

• ✅ Builds successfully with existing CMake configuration
• ✅ Code follows project style guidelines (clang-format applied)
• ✅ Logic validation confirms correct origin detection with appropriate tolerance
• ✅ No breaking changes to existing API

The world axis lines will be most visible in plots where axis ranges span across the origin, providing clear visual reference to the coordinate system center.

Fixes #108.

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[brenocq]

It is a bit hard to know which lines are "major tick" lines and which ones are "origin" lines. I just checked matplotlib and they don't have a special line color for major ticks. I think it would be better for all lines to have the same color (just like ImPlot), and only have "origin" lines with a special color to stand out.

[Copilot]

It is a bit hard to know which lines are "major tick" lines and which ones are "origin" lines. I just checked matplotlib and they don't have a special line color for major ticks. I think it would be b...

You're absolutely right! I've simplified the approach to match ImPlot and matplotlib behavior. Now all regular grid lines use the same color (no distinction between major/minor), and only origin lines have special styling to stand out. This provides a cleaner visual hierarchy with just two tiers: regular grid lines (60% opacity) and world axis lines (90% opacity). Fixed in commit 19cc070.