fix: SELECT TYPE dispatch logic scattered throughout business logic violates SOLID principles

[krystophny]

Problem

The codebase contains extensive SELECT TYPE dispatch logic scattered throughout business logic in fortplot_figure_core.f90, violating the SOLID principles, specifically:

• Open/Closed Principle: Business logic must be modified to add new backends
• Dependency Inversion: High-level modules directly depend on concrete backend types
• Single Responsibility: Figure module handles both orchestration and backend-specific rendering

Evidence

Found 14 instances of backend-specific type checks across 3 modules:

• fortplot_figure_core.f90: 8 instances
• fortplot_legend.f90: 4 instances
• fortplot_animation.f90: 2 instances

Examples include:

• Direct type checking for png_context, pdf_context, ascii_context
• Backend-specific rendering logic mixed with business logic
• Conditional execution based on backend type

Impact

• Adding new backends requires modifying core business logic
• Tight coupling between high-level and low-level modules
• Difficult to test business logic in isolation
• Violates architecture principle: 'NO SELECT TYPE IN BUSINESS LOGIC'

Required Fix

1. Move all backend-specific logic to backend implementations
2. Use polymorphic interfaces to hide backend differences
3. Configure backend behavior at initialization time
4. Remove all SELECT TYPE from business logic modules

Affected Files

• src/fortplot_figure_core.f90
• src/fortplot_legend.f90
• src/fortplot_animation.f90

[krystophny]

✅ COMPLETED: All 14 SELECT TYPE violations successfully eliminated from business logic modules.

Technical Achievement:

• Complete SOLID principle compliance achieved for backend polymorphism
• All business logic now uses polymorphic method calls instead of SELECT TYPE dispatch
• Polymorphic architecture enables clean backend extension without modification

Files Updated:

• : 8 SELECT TYPE blocks → polymorphic method calls
• : 4 SELECT TYPE blocks → polymorphic method calls
• : 2 SELECT TYPE blocks → polymorphic method calls

Verification:
All SELECT TYPE statements eliminated from business logic modules as confirmed by comprehensive testing.

Resolved in PR #152 with full CI validation across all supported Fortran compilers.

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