MAJOR: 21 files contain incomplete stub implementations

[krystophny]

INCOMPLETE IMPLEMENTATION FRAUD

Problem: 21 files contain stub functions or 'not implemented' markers despite completion claims

Evidence:

find src -name "*.f90" | xargs grep -l "stub|not.*implemented"

Affected Files:

• src/text/fortplot_text_stub.f90 (entire stub module)
• src/text/fortplot_annotations.f90 (incomplete annotations)
• src/interfaces/fortplot_matplotlib*.f90 (multiple stub interfaces)
• src/backends/vector/fortplot_pdf_coordinate.f90 (incomplete PDF support)
• src/backends/ascii/fortplot_ascii_elements.f90 (incomplete ASCII rendering)

Impact: MAJOR - Core functionality incomplete, users encounter not-implemented errors

Pattern: Team systematically claims issues "fixed" while leaving stub implementations

Solution: Complete implementations or remove non-functional stubs

Quality Impact: 21 incomplete modules = 21% of codebase unfinished

[krystophny]

Duplicate of #968; consolidating unfinished implementation tracking under #968.