Handle broad visitor callback annotations

[jeet-dekivadia]

Fixes #18.

_VisitorFuncWrapper uses the first callback parameter annotation as an optional runtime filter. Concrete node annotations work, but callbacks with no annotation silently skip every node because inspect.Parameter.empty is treated as a filter value. Callbacks annotated as Any raise TypeError because typing.Any cannot be used with isinstance().

This normalizes both cases to the existing visit-all path while leaving concrete typed callbacks unchanged. The regression covers an unannotated callback and an Any callback against the same expression tree.

Validation:

• git diff --check
• .venv/bin/python -m pytest test/stdlib/acceptance_test.py::test_broad_visitor_functions_visit_all_nodes
• uv format --diff
• uv run --all-extras --dev pytest
• uv run --all-extras --dev mypy heracles