feat(python): support CPython 3.14 and free-threaded builds

[muthmann]

Summary

• upgrade the Python bindings to PyO3 0.29 and numpy 0.29
• support standard and free-threaded CPython 3.14 without re-enabling the GIL
• replace removed Python::allow_threads calls with Python::detach
• add CPython 3.14 and 3.14t CI coverage and release wheels
• test free-threaded imports, independent parallel decoders, and shared read-only event access
• document the supported concurrency model and raise the Python binding MSRV to Rust 1.83

User impact

Users can install interpreter-specific wheels for both CPython 3.14 variants. On free-threaded CPython, importing evt3 keeps the GIL disabled. Independent decoder instances can run concurrently; shared NumPy arrays are supported for concurrent read-only access.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets -p evt3-core -p evt3-cli -p evt3-python -- -D warnings
• cargo test --workspace
• cargo check -p evt3-python with Rust 1.83
• CPython 3.9 synthetic suite: 18 passed
• CPython 3.14 synthetic suite: 18 passed
• CPython 3.14t synthetic and free-threading suite: 21 passed
• built and inspected cp314 and cp314t release wheels
• validated the CI and release workflow YAML

Closes #3