fix(report): type matplotlib handles

[Minghan2005]

Summary

• add type-only matplotlib Artist and Figure imports
• type the layer-chart legend handles and PNG serializer precisely
• retain lazy runtime matplotlib imports so the core package remains dependency-free

Validation

• core environment: mypy src
• matplotlib environment: mypy src, ruff check ., ruff format --check ., pytest -q (51 passed)

Closes #2

[Minghan2005]

I’ve rebased this branch onto the latest main and marked the PR ready for review. Local validation passes (ruff, mypy, and 51 tests). The new Actions run is waiting for maintainer approval; could you please approve it when convenient?