adapt composer to automatically generate inline defs in header-only mode. Add test cases.

src/pystencilssfg/composer/basic_composer.py

src/pystencilssfg/composer/class_composer.py

src/pystencilssfg/context.py

src/pystencilssfg/emission/file_printer.py

src/pystencilssfg/ir/entities.py

tests/generator_scripts/index.yaml

...s/generator_scripts/source/ComposerHeaderOnly.harness.cpp

tests/generator_scripts/source/ComposerHeaderOnly.py