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. The floor moved from 3.10 to 3.11 because the test toolchain no longer resolves on 3.10 (Hypothesis 6.170.12 needs 3.11), and the whole acmt001 suite moved to 3.11 with it in 0.0.14. No distro-system Python is supported.