fix: preserve interpolation through construction and with_extremes()

[matthiasschabel]

Two ways a colormap's interpolation mode goes missing.

The constructor writes into stops it does not own. The last line of Colormap.__init__ sets the requested mode on the ColorStops it was handed (src/cmap/_colormap.py:309), and that object usually belongs to the caller: _parse_colorstops returns a ColorStops argument unchanged (:1509), and the isinstance(value, Colormap) branch takes value.color_stops directly (:300).

base = Colormap(["red", "blue"], interpolation="nearest")
Colormap(base, interpolation="linear")   # discard the result

base.interpolation          # "nearest"
base.lut(4)                 # a smooth ramp: it renders linear now

The source keeps reporting "nearest" while rendering "linear", so the object contradicts itself and nothing raises.

with_extremes() drops the mode. It forwards name and category but not interpolation, and _norm_interp(None) is "linear", so one call converts a discrete colormap to a continuous one. An ordered class scheme like colorbrewer:Blues_9 starts blending between classes after with_extremes(bad=...).

The fixes are a shallow copy carrying its own mode, made only when the mode actually differs, and one added argument in with_extremes.

Shallow rather than rebuilt from scratch: a ColorStops can be backed by a user lut_func, and reconstructing would call that callable a second time over 256 values. Callables are part of the public ColormapLike API with no purity requirement, so a second call is neither free nor necessarily harmless.

A related issue probably requiring maintainer input: with_extremes also drops identifier. It's unclear whether an omitted bad/under/over should clear the existing one or leave it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.57%. Comparing base (8040ef7) to head (6f5cf96).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
+ Coverage   95.56%   95.57%   +0.01%     
==========================================
  Files         168      168              
  Lines        2186     2192       +6     
==========================================
+ Hits         2089     2095       +6     
  Misses         97       97              

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