Add test for gradient cache-miss branch (100% coverage)

[kdw503]

Summary

• Adds a single test that calls Interpolations.gradient at a location with a different center than the cached one, forcing the cache-update branch (lines 189–193) to execute.
• Coverage increases from 91.1% → 100%.

Test plan

• CI passes with all 41 tests green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (810f208) to head (e3f0d18).

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #18      +/-   ##
===========================================
+ Coverage   91.07%   100.00%   +8.92%     
===========================================
  Files           1         1              
  Lines          56        56              
===========================================
+ Hits           51        56       +5     
+ Misses          5         0       -5     

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