test: add sphinx-build integration coverage

[BlocksecPHD]

Summary

• add an integration test that runs a real sphinx-build HTML build for the .. callgraph:: directive
• use a tiny fake dot executable so the test exercises Sphinx's Graphviz rendering path without depending on a system Graphviz install in CI
• assert that the generated HTML references the rendered SVG output and keeps the zoomable callgraph behavior wired up

Closes #114.

Testing

• ruff check .
• pytest tests/test_sphinx.py -q
• pytest tests/test_coverage.py tests/test_sphinx.py tests/test_writers.py -q
• pytest tests/ -v

Notes

• the fake dot stub is intentional: it keeps the test focused on the Sphinx integration boundary instead of the external Graphviz binary implementation

[Technologicat]

Looks good.

Merged with minor edits (capture stderr from subprocess; run sphinx-build via python -m sphinx; and add Windows support for the stub).

Thanks for the contribution!

— Technologicat