test(logAxis): add visual test HTML for asinh/symlog log mapping

Adds test/log-mapping.html with 8 chart scenarios covering the new
logMapping: 'asinh' and logMapping: 'symlog' axis options:

1. Standard log baseline (regression guard)
2. asinh, positive-only (visual comparison with standard log)
3. asinh, mixed [-1000, 1000] — primary use case, zero-crossing data
4. asinh, all-negative extent
5. asinh with logBase: 2 (ticks at powers of 2)
6. asinh scatter with P&L-style mixed-sign data
7. symlog, mixed [-1000, 1000] (side-by-side with scenario 3)
8. symlog with explicit logLinearWidth: 1 (linear segment visible)

Each chart title lists what to verify, serving as an inline visual
checklist for reviewers.

Author: netscout-mthorn