feat: enhance text and annotation stub implementations

[krystophny]

Summary

Replace stub implementations in fortplot_text_stub.f90 with proper functionality:

• Implement text() subroutine for text annotations
• Implement annotate() subroutine for annotations with arrows
• Add support for coordinate types, font sizing, rotation, alignment
• Add support for bounding boxes and arrow styling

Acceptance Criteria

• Remove log_warning calls and implement actual functionality
• Support all parameter options for text positioning and styling
• Integrate with existing rendering backends (ASCII, PDF, raster)
• Update module name from fortplot_text_stub to fortplot_text

Priority

Low - Enhancement for text annotation features

Related to issue #467

[krystophny]

NON-ACTIONABLE: Feature enhancement requests are not defects. Enhancement belongs in PRODUCT_BACKLOG planning, not GitHub issues.