fix(ewa): use the requested fill value instead of hardcoded NaN

[Manny7717]

What

fornav() resolves a fill value from the input data (NaN for floats, -999 for integers, or the caller's fill argument) but then passed hardcoded np.nan as both the input and output fill to the C resampler (_fornav.fornav_wrapper). Two consequences (reported in #689):

• Integer swath data cannot be resampled — NaN cannot be written into an integer output array.
• A user-specified fill (e.g. fill=0) was ignored for float data: uncovered pixels still got NaN.

Change

Pass the resolved fill value as both input and output fill to fornav_wrapper. Default behavior is unchanged (float data with fill=None still gets NaN, matching the documented default); only fill-specified and integer paths change. The dask EWA resampler already forwarded its fill value and is unaffected.

Tests

• New test_fornav_integer_swath_with_fill: int8 swath + fill=0 resamples without error; output contains only data and fill values.
• New test_fornav_fill_value_used_in_output: float32 swath + fill=0 — uncovered pixels are 0, no NaN anywhere.
• Both tests fail on the previous implementation (verified) and pass with the fix.
• pytest test_ewa_fornav.py test_dask_ewa.py → 89 passed, 20 skipped (pre-existing skips). ruff clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.69%. Comparing base (2e7a3c8) to head (5cfd105).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #741      +/-   ##
==========================================
+ Coverage   93.68%   93.69%   +0.01%     
==========================================
  Files          89       89              
  Lines       13715    13739      +24     
==========================================
+ Hits        12849    12873      +24     
  Misses        866      866              

Flag	Coverage Δ
unittests	93.69% <100.00%> (+0.01%)	⬆️

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