Add fused detection postprocessing

[GiraudJules]

Summary

Add a fused detection postprocessing path that remaps boxes, clips them, filters them, and runs class-aware hard or soft NMS in one Rust call.

Changes

• add postprocess_detections in the Rust bbox module
• support current-space, resize, and letterbox remapping modes
• expose postprocess_detections(...) through the Python NumPy API
• extend Rust tests and the Python smoke test with remap-aware detection cases

Why

This reduces Python overhead between model outputs and final detections, which is one of the core goals of rusty-cv.

Validation

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