refactor: Use padding built into Interpolations.jl instead of rolling our own

[icweaver]

pad_edges (which copied the input into a padded array with replicated borders) is replaced by Interpolations.extrapolate with a padding = Flat() keyword on interpolator. Sampling semantics are identical for orders 0 and 1; order 2 changes only within the existing test tolerances and now matches the coordinate-clamping approach modern astropy reproject uses (astropy/reproject#352). This also removes a ::Matrix restriction, so any AbstractMatrix (e.g., an AstroImage) can flow through the interpolation core.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.11%. Comparing base (57b10e6) to head (15a0b1f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
- Coverage   94.68%   94.11%   -0.57%     
==========================================
  Files           4        4              
  Lines          94       85       -9     
==========================================
- Hits           89       80       -9     
  Misses          5        5              

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