Fix clean imports on Python 3.14

[aviadr1]

Importing winuvloop on Python 3.14 previously resolved deprecated backend policy APIs and emitted an AbstractEventLoopPolicy warning before callers used the package. This PR makes those compatibility exports lazy, so supported run() and loop-factory usage imports cleanly while explicit access to deprecated APIs continues to follow the backend's behavior.

It also keeps winuvloop's public API aligned with uvloop and winloop: install and EventLoopPolicy remain exported through Python 3.15 and leave __all__ and the typing stub when event-loop policies are removed in Python 3.16.

Changes

• Stop eagerly resolving backend install and EventLoopPolicy attributes.
• Preserve both compatibility names lazily through Python 3.15.
• Remove the deprecated names from __all__ and type declarations on Python 3.16+.
• Run the clean-import regression in a fresh interpreter so backend caching cannot hide the warning.
• Add cross-platform lifecycle coverage for both sides of the Python 3.16 boundary.
• Document the compatibility behavior in the changelog.

Evidence

The lifecycle regression fails against the previous PR revision because both deprecated names remain in __all__ for simulated Python 3.16.

After the fix:

• GitHub Actions: all 7 jobs passed.
• Tests: 17 passed on Linux with Python 3.8, 3.12, and 3.14.
• Cross-platform CI: Linux, macOS, and Windows passed.
• Ruff lint and format checks passed.
• Ruff preview F822 undefined-export check passed.
• Source distribution and wheel built successfully; Twine metadata checks passed.

Closes #37.