Add Python 3.14 support to CI and package metadata

[PandaHUN777]

Closes #10

Adds Python 3.14 to the Linux verification matrix and to the package classifiers. The existing Windows verification job stays on Python 3.13.

The contributor documentation now reflects Linux coverage through Python 3.14, and the Unreleased changelog notes the newly declared support.

The existing uv.lock already supports Python >=3.11 and has a resolution range covering Python 3.14, so no dependency or lockfile changes were needed.

No admission decisions, validation behavior, report contents, or runtime code are changed.

I have verified the full verification path on Python 3.14 inside an isolated PRTEST environment: uv sync,
uff check ., mypy, pytest --cov (198/198 passed, 97.86% coverage), wheel build, and the installed-wheel CLI smoke check (check_distribution.py) all passed cleanly on CPython 3.14.4.

[NingyuSUN]

Thanks for picking up #10, and for the careful write-up!

Verified: CI now passes on all jobs, including the new verify (ubuntu-latest, 3.14); the classifier is added; keeping Windows on 3.13 is fine and clearly stated. All three done-criteria of #10 are met.

Two small things before merging:

1. CHANGELOG placement. Since you opened this, 0.7.0 was released and its "Unreleased" section was renamed to "0.7.0". As it stands, your line merges without a conflict but lands inside the published 0.7.0 section. Please move it under the new ## Unreleased heading at the top of CHANGELOG.md (added in #16).
2. docs/ENGINEERING.md still says "CI runs on Linux/Python 3.11–3.13 …" — please update it to 3.14 as you did in CONTRIBUTING.

Happy to merge once those are in. (CI ran against main as of when the PR was opened; it will re-run when you update the branch.)

[PandaHUN777]

Thanks for the review, @NingyuSUN!

I have updated the branch with the requested documentation adjustments:

• Moved the CHANGELOG entry under the top-level ## Unreleased section.
• Updated docs/ENGINEERING.md so the documented Linux CI Python range reflects 3.11?3.14.

Validated locally with:

• uv run --frozen ruff check .
• uv run --frozen mypy
• uv run --frozen pytest --cov --cov-report=term (230 tests passed, 98.09% coverage)
• uv build
• uv run --isolated --no-project --with ./dist/*.whl python tools/check_distribution.py
• git diff --check