Expose combined __annotations__ on composed functions (#496)

[az51826295-sys]

Compose already synthesizes __signature__ (parameters of the first function applied, return annotation of the last), but it had no __annotations__, so typing.get_type_hints() and tools that read __annotations__ directly saw nothing on a composed callable.

This adds an instance property that builds the same view:

def f(x: int) -> str: ...
def g(y: str) -> float: ...
compose(g, f).__annotations__          # {'x': int, 'return': float}
typing.get_type_hints(compose(g, f))   # {'x': int, 'return': float}

Functions without annotations contribute nothing; callables with no __annotations__ at all (e.g. str, arbitrary objects) are handled.

Scope: this is a small runtime step toward the static typing discussed in #496 — the composed callable now reports Callable[[A], C] for compose(g, f) with f: A -> B, g: B -> C. It does not add stubs or annotate the library itself; happy to split or drop it if you'd rather approach #496 differently.

Tests: test_compose_annotations (combined view, unannotated pieces, get_type_hints, consistency with __signature__). Full suite incl. doctests passes locally (pytest --doctest-modules toolz/; the only failure is test_has_version, which needs an installed distribution and fails identically on master in my environment).

Authored with the help of an AI agent (Rookery Alpha), human-reviewed.