fix: preserve colormap state through reversed(), with_extremes(), pickle, and as_dict()

[matthiasschabel]

Follow-up to the four preservation questions on #151. Depends on #151: the base commits here are
that PR's, and the diff shrinks to the preservation commit alone once it lands.

None of the four channels preserved the extreme colors, and two of them dropped the interpolation
mode as well. Verified on main before this change:

• reversed() passes only stops, name, and category, so a nearest colormap comes back linear
  with no extremes.
• with_extremes() clears anything not repeated in the call.
• __reduce__ carries only color_stops, so pickle.loads(pickle.dumps(cm)) == cm is already
  False for any colormap with bad set.
• as_dict() has no keys for interpolation or the extremes, and it is what both the pydantic
  serializer and _json_encode emit.

Following matplotlib 3.11 where it has a position:

• reversed() swaps under/over and neg_inf/pos_inf, which name the ends they extend, and
  preserves the rest.
• with_extremes() preserves anything not passed. Clearing a single color now means constructing
  a new Colormap, the same limitation matplotlib has. This is the one backward-incompatible
  change here.
• __reduce__ carries the full constructor state.
• as_dict() gains optional keys, written only when they hold non-default state, so existing
  payloads are unchanged.

Two changes that are more than preservation:

• Colormap("x_r") now swaps the catalog record's under/over, so it agrees with
  Colormap("x").reversed(). napari:HiLo is the only affected entry. An explicit
  under=/over= argument still lands on the end the caller named.
• The pydantic serializer emitted a catalog colormap's qualified name unconditionally, so
  Colormap("viridis", under="red") came back as plain viridis, and Colormap("viridis_r") came
  back unreversed because info is looked up under the stripped name. It now emits the name only
  when the name alone rebuilds the same as_dict(). __eq__ is not usable as that guard: it
  ignores name, identifier, and category, and ColorStops.__eq__ compares with np.allclose, so
  a cmap_kwargs={"start": 0.500001} cubehelix compares equal to the default one.

What I left alone, and why:

• Pickle passes color_stops as the object rather than as_dict()'s samples, so a colormap
  backed by a lut function keeps the function. Routing pickle through as_dict() shifted
  lut(17, gamma=2) of a parametrized cubehelix by 2e-5.
• A round trip through the dict form does not restore info, which is set only from a string or
  another Colormap, so an unpickled catalog colormap serializes as a dict rather than as its
  name. Fixing that means putting the qualified name in value, which would change existing
  payloads for every catalog colormap.
• identifier follows the name: reversed() and shifted() rename, so it re-derives;
  with_extremes() does not, so it is preserved. One consequence: cm.reversed().reversed()
  restores the name but not an explicitly supplied identifier. Say if you would rather it were
  carried through.
• The converters still have no representation for the finer classes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.95%. Comparing base (f0a4aec) to head (313a7d9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   95.72%   95.95%   +0.22%     
==========================================
  Files         168      168              
  Lines        2197     2247      +50     
==========================================
+ Hits         2103     2156      +53     
+ Misses         94       91       -3     

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