Add step parameter to boundaries() and mort2polygon()

[espg]

Summary

• Add configurable boundary resolution via the healpix crate's path_along_cell_edge() method
• step=1 (default) returns the original 4 corners; step=32 returns 128 boundary points that accurately trace curved cell edges
• Fixes polygon distortion near the poles where 4-corner projections to EPSG:3031 underestimate the true HEALPix cell area by ~3.5km

Changes

Rust (geo2mort.rs): New boundaries_step_scalar() function using Layer::path_along_cell_edge(pixel, Cardinal::S, true, n_segments_by_side), with vertex ordering rolled to match healpy convention.

PyO3 (lib.rs): rust_boundaries() gains a step parameter (default 1). Step=1 takes the fast original path; step>1 dispatches to the new function. Output shape: (3, 4*step) scalar, (N, 3, 4*step) array.

Python (_healpix.py, tools.py): boundaries(nside, pixel, step=1) and mort2polygon(morton, step=1) pass through the parameter.

[codspeed-hq]

Merging this PR will not alter performance

✅ 14 untouched benchmarks

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_{Comparing feature/boundaries-step (3a422cd) with main (9763868)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.82%. Comparing base (9763868) to head (3a422cd).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
- Coverage   86.49%   85.82%   -0.67%     
==========================================
  Files          13       13              
  Lines        1770     1771       +1     
==========================================
- Hits         1531     1520      -11     
- Misses        239      251      +12     

Flag	Coverage Δ
unittests	85.82% <100.00%> (-0.67%)	⬇️

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Files with missing lines	Coverage Δ
mortie/_healpix.py	97.05% <100.00%> (ø)
mortie/tools.py	77.24% <100.00%> (-3.54%)	⬇️

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