fix: remove redundant stateful_streamplot example

[krystophny]

Summary

• Remove duplicate stateful_streamplot example that was functionally identical to streamplot_demo
• Both examples used the same 20x20 grid, same circular flow field, same API calls
• Only difference was title text and directory structure
• Clean up all documentation references, build system entries, and source file mappings

Changes Made

• Deleted: example/fortran/stateful_streamplot/ directory and all contents
• Deleted: doc/examples/stateful_streamplot.md documentation
• Updated: Documentation index files to remove references
• Updated: Makefile to remove output directory creation
• Updated: DESIGN.md to remove from example listings
• Updated: Source files (fortplot_doc_examples.f90, fortplot_doc_media.f90) to remove mappings

Validation

• All references to stateful_streamplot have been completely removed
• streamplot_demo remains as the single comprehensive streamplot example
• Build system updated to match new structure

Test Plan

• Verify build system works without stateful_streamplot references
• Confirm documentation no longer references removed example
• Ensure streamplot_demo still functions as expected

Fixes #336

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/fortplot_doc_examples.f90	0.00%	2 Missing ⚠️

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