refactor: complete symlog tick generation implementation

[krystophny]

The symlog tick generation functions in fortplot_axes.f90 are currently stub implementations that need to be completed.

Current State

• add_negative_symlog_ticks() is a stub
• add_linear_symlog_ticks() is a stub
• add_positive_symlog_ticks() is a stub

Needed

• Implement proper tick generation for symlog scale regions
• Add tests for symlog edge cases
• Ensure smooth transitions between regions

Found during code review of PR #341

[krystophny]

Fixed in PR #369 - symlog tick generation implementation completed with proper exponential positioning and overlap handling.