Add Rust CI workflow and stub generation
- Add .github/workflows/rust.yaml for Rust CI (fmt, clippy, nextest)
- Add mise tasks: stubs:generate, stubs:check, stubs:typecheck
- Add mise tasks: ci:rust, ci:coglet for local CI simulation
- Add scripts/generate_stubs.py to auto-generate coglet.pyi from module
- Update coglet.pyi with all module exports (classes + internal functions)
CI runs:
- Pure Rust checks (fmt, clippy, tests) without Python
- coglet-python checks on Python 3.9/3.12/3.13 (build, stub check, typecheck)
