fix: assign the "over" color to napari's high_color, not nan_color

[matthiasschabel]

Colormap.to_napari() checks for napari's high_color field but assigns the over color to nan_color (src/cmap/_external.py:115), introduced in #103.

Two effects:

• high_color is never set, so values above the range fall back to napari's default instead of the requested over color;
• the assignment runs after the bad_color branch, so an explicitly configured bad color is silently replaced by the over color.

One-line fix plus a regression test asserting that low_color, high_color, and nan_color each receive their own color. The test requires napari >= 0.6.1, where those three fields were added — below that the converter correctly skips them.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.60%. Comparing base (8040ef7) to head (ba7129b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   95.56%   95.60%   +0.04%     
==========================================
  Files         168      168              
  Lines        2186     2186              
==========================================
+ Hits         2089     2090       +1     
+ Misses         97       96       -1     

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[tlambert03]

thanks