Require the future-annotations import with ruff

[imnasnainaec]

Every module under src/ opens with from __future__ import annotations
except _extras.py,
which needs nothing from it, so its absence was invisible. Ruff's isort
required-imports enforces what was being eyeballed, and supplies the missing
import.

The rule is scoped to the package. Tests and scripts mostly do without the
import, and that is their own convention rather than a lapse in this one, so
both are exempted from I002 rather than gaining 15 no-op imports.

What is actually being decided

Whether the convention is worth a linter at all. That is a preference, settled
by argument rather than by any test, which is why a change this small is worth
its own review.

Nothing behavioural turns on it either way. The added import is a no-op in
_extras.py — requires-python is already >=3.11, where every annotation in
that module evaluates fine without it — so what is on offer is the guarantee
that the next module cannot quietly omit it, against a cost of one config
block.

ruff check, ruff format --check and mypy are green.

🤖 Generated with Claude Code

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This change is 

[jasonleenaylor]

This codifies an invariant the package already depends on rather than introducing a style preference -- the TYPE_CHECKING-only imports in public signatures resolve at runtime only because annotations are strings. Worth making enforceable.

I checked the one thing that could have bitten: _extras.py is the only file newly switching to string annotations, and the only runtime annotation introspection in the repo is tests/test_public_api.py, which reads raw PEP 563 strings deliberately. No exposure.

One nit inline.

This review was assisted by Claude Opus 5.

[jasonleenaylor]

@jasonleenaylor reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on imnasnainaec).