fix: don't mutate the source stops in ColorStops.reversed()

[matthiasschabel]

ColorStops.reversed() corrupts the object it is called on (src/cmap/_colormap.py:1164).

self._stops[::-1] is a view, so rev_stops[:, 0] = 1 - rev_stops[:, 0] writes through to the source: every position p becomes 1 - p while the colors stay where they were. A red/green/blue ramp at [0, 0.5, 1] is left holding [1, 0.5, 0] — descending stops the constructor itself rejects with "Color stops must be in ascending position". __init__ stores arrays uncopied (:918), so the returned object aliases that buffer too.

cmap = Colormap(["red", "green", "blue"])
cmap.reversed()
cmap.lut(5)[:, :3]  # -> red, red, blue, blue, blue  (green is gone)

Two .reversed() calls on the same object also disagree, since the first corrupts the input to the second.

Copying the reversed view before rewriting positions fixes both halves, and matches shifted() twelve lines below, which already copies before rewriting positions.

Nothing exercised this: the existing reversal assertions use the reversed() builtin (__reversed__), a different method, and cmap's two internal callers discard the corrupted source immediately.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.70%. Comparing base (2c4b86e) to head (c21861e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #147   +/-   ##
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  Coverage   95.70%   95.70%           
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  Files         168      168           
  Lines        2188     2188           
=======================================
  Hits         2094     2094           
  Misses         94       94           

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[tlambert03]

good catch thanks