Decide the Python floor: justify >=3.14.4 or relax it

[eaitbrahim]

pyproject.toml sets requires-python = ">=3.14.4" and .python-version pins 3.14.4. Python 3.14 is very new: many contributors will not have it, several distributions do not package it yet, and some CI images lag. This is a real barrier that will silently cost contributors — they will try, fail to build, and leave without opening an issue.

Decide, with evidence

• Establish what actually requires 3.14 — a specific language or stdlib feature, or nothing in particular. Check for match ergonomics, Decimal behaviour, typing syntax, free-threading assumptions
• If a real dependency exists: keep the floor and say so prominently in README and CONTRIBUTING.md, with the one-line reason and a uv python install instruction
• If nothing requires it: relax to the lowest version the test suite passes on, and add that version to CI so it stays true

Acceptance

• Decision recorded with its reason, not just changed
• If the floor stays, a contributor hitting it gets a clear message rather than a confusing resolver error