Toolchain policy

Python 3.11 is the floor (python = ">=3.11,<4.0" in pyproject.toml), with Linux CI on 3.11, 3.12, 3.13 and 3.14. The container image runs 3.14. Raising the floor is a maintainer decision, recorded in the changelog, and moves with the pacs008 core library. No distro-system Python is supported.