chore(python): relax the floor to >=3.11, measured, with a CI leg to keep it true

[eaitbrahim]

Closes #283

requires-python = ">=3.14.4" said "the interpreter I develop on", not "the interpreter the
code needs" — and 3.14 is new enough that contributors who lack it fail to build and leave
without opening an issue.

The measurement (the decision's evidence, per the issue):

Python	full suite
3.13	2,788 passed / 1 skipped — identical to 3.14
3.12	2,788 passed / 1 skipped
3.11	2,788 passed / 1 skipped
3.10	fails collection: ImportError: cannot import name 'assert_never' from 'typing'

Decision: nothing requires 3.14. The floor is set to >=3.11 — the lowest version the
suite passes on — with typing.assert_never (3.11+) as the one concrete binding constraint.
Development stays pinned to 3.14.4 via .python-version.

What changes

• requires-python = ">=3.11" in all six distributions; uv.lock re-resolved.
• ci.yml's test job becomes a 3.11 + 3.14 matrix, so the floor stays a measured claim.
  Both legs stay inside the test job on purpose: the matrix fans out the required status
  context rather than renaming it, so every leg gates the merge — the same lesson the job's
  own comments record for mypy (ci: gate on mypy, so #266's ungating cannot quietly come undone #268).
• mypy python_version = "3.11" — check against the floor, not the dev pin.
• README quickstart and CONTRIBUTING state 3.11 and the reason ("a contributor hitting the
  floor gets a clear message rather than a confusing resolver error" — low floor, stated
  plainly).
• tests/test_python_floor.py (red first) pins the decision's four halves: one floor in every
  distribution, the binding feature still imported (if assert_never ever disappears, the
  floor can drop — re-measure), the CI leg present, and the docs stating it.

Gates: pytest 2,798 passed / 1 skipped on both 3.14 and 3.11; ruff and mypy clean.