Add draft stub generation and Action typing

[james-d-mitchell]

Dynamic wrapper methods and native bindings lose useful type information during ordinary stub generation. This adds a reusable generator that combines pybind11-stubgen discovery with source annotations, generic bases, and documented signatures.

• Require an explicit module and print a single stub to stdout, keeping diagnostics on stderr. Files are written only with --output-dir; replacing existing files requires --force.
• Default extension generation to publicly exported native classes and their typing dependencies. --no-public-only requests the full extension, and --strict stops on backend diagnostics.
• Add sample Action and BMat8 helper-module stubs, make Action generic in its element and point types, and correct the copy_cxx_mem_fns class annotation.
• Document usage and cover generation, public-class selection, output handling, and mypy consumer checks.

Generated stubs remain drafts: some native signatures still need manual correction. The mypy tests cover Action and an isolated BMat8 section, not the complete native stub. This does not add a py.typed marker.

Validation:

• uv run --offline --no-sync --with pybind11-stubgen==2.5.5 --with mypy==2.3.1 pytest tests/test_generate_stubs.py tests/test_action.py tests/test_bmat8.py -k 'not test_action_transf': 58 passed, 1 deselected. The excluded test enumerates a large transformation action.
• Both pre-commit hook stages passed on all changed files; explicit Ruff checks and formatting passed for the generator as well.
• Confirmed tests use the package and built extension from this checkout. No C++ rebuild was needed; documentation builds and the full test suite were not run for this Python typing/tooling change.

AI disclosure: OpenAI Codex helped prepare the implementation, tests, documentation, commit message, and this PR description, and ran validation. The commit includes Assisted-by: Codex <codex@openai.com> and Co-authored-by: Codex <codex@openai.com> trailers.

[james-d-mitchell]

This is a partial attempt (or start of an attempt) to resolve #476