Add segmentation mask geometry helpers

[GiraudJules]

Summary

Add the first segmentation-oriented geometry helpers and expose them to Python.

Changes

• add a new mask module in Rust
• add nearest-neighbor mask resize
• add mask letterbox and unletterbox helpers
• add binary mask thresholding
• add mask-to-box extraction
• expose the corresponding NumPy APIs in Python
• extend Rust tests and the Python smoke test

Why

This broadens rusty-cv beyond detection and moves more common Python segmentation geometry work into Rust.

Validation

• cargo fmt --check
• cargo test
• cargo check --features python
• ../../.venv/bin/maturin develop --features python
• ../../.venv/bin/python scripts/python_smoke_test.py