feat: initialize standalone image-filter

Rename the distribution to image-filter and imports to image_filter.
Keep medical fixtures local and document their setup for fresh clones.
Include composite pipelines, consistent examples, and class diagrams.

.gitignore

@@ -0,0 +1,20 @@
+/.pytest_cache/
+/build/
+/dist/
+*.egg-info/
+__pycache__/
+*.py[cod]
+
+/example-output/
+/data/
+.DS_Store
+.venv/
+venv/
+.env
+.env.*
+.coverage
+htmlcov/
+.mypy_cache/
+.ruff_cache/
+*.log
+/filter-qa.png