✨ feat: Add Python 3.15 support

[ShigureNyako]

Changes

• Add regular Python 3.15 to the existing test matrix and classifiers. Keep Python >=3.12 and all existing coverage.
• Raise watchfiles from 1.1.0 to the minimum needed 1.2.0, not 1.3.0, and update only its lock entry.

Why the dependency change: 1.1.1 still has no usable 3.15 wheels, and its source build rejects 3.15 because PyO3 0.26 supports up to 3.14. Version 1.2.0 supplies cp315 wheels; its binary-only installation, import, CLI, and all 20 tests pass on 3.15rc2.

Per review, the final diff is only three files: existing matrix, pyproject.toml, and uv.lock. No added tests, README text, interpreter/GIL checks, Rust setup, or extra platform/wheel CI remains. The lockfile changes only watchfiles artifacts/version and the root requirement.

Validation

• Head 4221176: all four Python test jobs and lint/format pass.
• Local locked runs on 3.12.14, 3.13.15, 3.14.7, and 3.15rc2: 20 tests pass each.
• Built-wheel install with watchfiles 1.2.0, CLI smoke, and 20 tests pass on 3.15rc2. Ruff, format, ty, actionlint, lockfile check, build, twine, and diff checks pass.

3.15t is deferred per review: no published watchfiles 3.15t wheels, and the previously investigated macOS 27 source-build import failure remains unresolved. This PR makes no 3.15t support claim.