feat: add isize geometry module

[lemmih]

Summary

Add an apfp::geometry::isize module to provide geometric predicates for isize coordinates.

Motivation

When integrating apfp into rgeometry, we found that isize is a commonly used coordinate type but there's no corresponding apfp::geometry::isize module. Currently we work around this by casting to i64:

impl PolygonScalar for isize {
  fn cmp_slope(p: &[Self; 2], q: &[Self; 2], r: &[Self; 2]) -> std::cmp::Ordering {
    use apfp::geometry::{Orientation, i64 as apfp_mod};
    let orient = apfp_mod::orient2d(
      &apfp_mod::Coord::new(p[0] as i64, p[1] as i64),
      &apfp_mod::Coord::new(q[0] as i64, q[1] as i64),
      &apfp_mod::Coord::new(r[0] as i64, r[1] as i64),
    );
    // ...
  }
  // ... similar for cmp_dist, cmp_vector_slope, cmp_perp_vector_slope, incircle
}

This works (since isize is at most 64-bit on all platforms), but it would be cleaner to have native isize support.

Proposed Solution

Add apfp::geometry::isize module with the same API as the other integer modules:

• Coord struct with isize components
• orient2d, orient2d_vec, orient2d_normal
• cmp_dist
• incircle

The implementation could either:

1. Delegate to i64 internally (simple, works everywhere)
2. Use cfg attributes to delegate to i32 or i64 based on platform pointer width (slightly more efficient on 32-bit platforms)

Option 1 is probably fine since 32-bit platforms are increasingly rare.