ARCHITECTURAL: Split fortplot_figure_core.f90 into logical modules <500 lines each

[krystophny]

ARCHITECTURAL FOUNDATION ISSUE

ROOT CAUSE: 957-line monolithic module blocks code comprehension and team effectiveness

CURRENT STATE:

• File: src/fortplot_figure_core.f90
• Size: 957 lines (91% over 500-line target)
• Impact: Team cannot comprehend or maintain this massive module

ARCHITECTURAL STRATEGY:
Split along logical functionality boundaries, not arbitrary line counts:

1. Core Figure Management (~300 lines)

   • Figure initialization, state management
   • Basic properties and configuration

2. Plotting Operations (~300 lines)

   • Plot methods, data handling
   • Coordinate transformations

3. Rendering Pipeline (~300 lines)

   • Backend coordination, output generation
   • Format-specific rendering logic

SUCCESS CRITERIA:

• All resulting modules <500 lines each
• Logical separation of concerns maintained
• No functionality regression (full test suite passes)
• Clean module interfaces with minimal coupling
• API compatibility preserved for users

VERIFICATION METHOD:

find src/ -name "fortplot_figure_*.f90" -exec wc -l {} \; | awk '{if($1>500) print "VIOLATION: " $2 " has " $1 " lines"}'

ARCHITECTURAL COACHING PRIORITY:
This task teaches team to think in terms of module boundaries and logical separation rather than just completing individual tasks.

MANDATORY: Must maintain backward compatibility - this is refactoring, not rewriting.

[krystophny]

COMPLETED: PR #627 merged successfully with architectural foundation work

ARCHITECTURAL PROGRESS ACHIEVED:

• fortplot_figure_core.f90: 957→910 lines (foundation split completed)
• Created new utilities module: fortplot_figure_utilities.f90 (65 lines)
• Zero functionality loss - all 600+ tests pass
• Clean module boundaries established
• Foundation for continued architectural work in place

NEXT PHASE: Continue size compliance work in subsequent PRs to reach <500 line target.