Make FrozenDict generic and fix its hashing

[ESultanik]

Stacked on #16.

FrozenDict was untyped, so index_type_map[4] gave Any and nothing about the mapping's key or value types survived to callers. It is now FrozenDict[K, V_co].

Written with TypeVar + Generic rather than PEP 695 class FrozenDict[K, V], since that syntax requires Python 3.12 and this package supports 3.10. The two are equivalent to a type checker; only the spelling and variance inference differ. V_co is covariant because the mapping is read-only.

Hashing

__hash__ had two problems. The isinstance(self._mapping, Hashable) branch was unreachable — dict sets __hash__ = None, so a dict is never Hashable — and the branch that actually ran hashed only self._mapping.keys(). Since Mapping.__eq__ compares values, two FrozenDicts with the same keys and different values were unequal but always collided:

hash(FrozenDict({'a': 1})) == hash(FrozenDict({'a': 2}))   # was True

Now hashes the items, falling back to the keys when a value is unhashable, which preserves the equal-implies-equal-hash requirement either way.

__init__ builds through an explicitly Any-typed local: keyword arguments can only contribute str keys, so inferring directly widened the key type to K | str and contradicted the declaration.

Verification

Tests go from one omnibus case to 13, covering mapping behaviour, immutability, hash/eq consistency, value-sensitive hashing, unhashable values, use as a dict key, and runtime parameterisation. ty is clean on the module.

🤖 Generated with Claude Code

https://claude.ai/code/session_01367hFob9sd4xpDmVT4uoFy