Pickle nonimportable Enums so Dataset.map(num_proc>1) and hashing work

[behroozazarkhalili]

Resolves #2643

Root cause

dill serializes a class it cannot import by its module path through _create_type, which rebuilds the class from type(name, bases, dict). That path cannot rebuild an Enum: the members, _value2member_map_, aliases, Flag boundary and slot descriptors are produced by EnumMeta.__new__, so an Enum defined in __main__ or inside a function fails to pickle. Dataset.map(num_proc>1) sends the shard function through multiprocess, whose ForkingPickler uses the same dispatch, so workers died unpickling; the fingerprint Hasher hit the same error and fell back to a random fingerprint.

Fix

• New Enum reducer in datasets/utils/_dill.py: rebuilds nonimportable Enums through their own metaclass with members, aliases (including Python 3.13+ unhashable value aliases), Flag boundary and slot state, then validates at dump time by reconstructing and comparing member state so a class that cannot be rebuilt faithfully raises at dump rather than producing a wrong class in the worker.
• Members and class-referencing hooks (__init__, __new__, _generate_next_value_, zero-argument super()) are restored after the class is memoized, so self-referencing Enums no longer hit dill's recursion guard.
• Lazily populated caches (_inverted_, negative-value lookups) are excluded from the serialized state, so the hash of an Enum does not depend on which lookups happened before.
• Explicit contracts win: class-, base- and instance-level __reduce_ex__/__reduce__, copyreg and dill.register reducers take precedence over the Enum reducer.
• Faithfulness checks use _name_/_value_ internals, treat NotImplemented and raising __eq__ as unknown rather than failure, and compare array values with array_equal, so nothing that upstream could pickle becomes unpicklable. Slot discovery runs once per class; dumps are linear in member count.
• The reducer is registered on datasets' Pickler and on multiprocess.reduction.ForkingPickler. Pool binds the shared ForkingPickler with no per-pool injection point; test_map_enum_requires_worker_dispatch shows a two-worker map fails without it. The effect on other multiprocess users is documented at the registration site.

Tests

41 new tests in tests/test_fingerprint.py: nonimportable Enum/IntEnum/StrEnum/Flag/IntFlag round-trips through dumps and ForkingPickler, real two-worker Dataset.map with a local Enum (fork and spawn contexts), aliases and NaN values, custom __new__/_missing_/__init__ hooks, lookup-cache independence of the hash, instance and registered reducers, numpy-valued members, undefined equality, raising public accessors, constructor replay (documented behaviour), large-Enum scaling, and dump-time failures for classes that cannot be rebuilt faithfully (date mixin, values changed after construction).