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

[matthiasschabel]

Depends on #151. Follow-up to the four preservation questions from that PR:

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().

Left unchanged:

• Pickle passes color_stops as the object rather than as_dict()'s samples, so a colormap backed by a lut function keeps the function.
• 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.
• 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 - not 100% sure this is the correct behavior.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.95%. Comparing base (f0a4aec) to head (4302481).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
+ Coverage   95.72%   95.95%   +0.23%     
==========================================
  Files         168      168              
  Lines        2197     2250      +53     
==========================================
+ Hits         2103     2159      +56     
+ Misses         94       91       -3     

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