chore: Python 3.12 is the floor, part A (#6)

The shared workflow tests on 3.12 and 3.13 by default and runs lowest on
3.12; the template declares requires-python >=3.12 with ruff and mypy on
3.12, and the tool itself needs 3.12. netOrk runs every driver on 3.12 in
its own containers.

check keeps accepting a 3.10 floor until every driver has moved; part B
raises it.

CHANGELOG.md

@@ -6,6 +6,11 @@ The format follows [Keep a Changelog](https://keepachangelog.com/), versions fol
 ## [Unreleased]
 
 ### Changed
+- Python 3.12 is the floor: the shared workflow tests on 3.12 and 3.13 by default and
+  runs `lowest` on 3.12, the template declares `requires-python = ">=3.12"` with ruff
+  and mypy targeting 3.12, and the tool itself needs 3.12. netOrk runs every driver in
+  its own containers on 3.12, so older versions are no longer tested. `check` keeps
+  accepting a 3.10 floor until every driver has moved.
 - `check` reports a license given as a TOML table or as a `License ::` classifier:
   setuptools refuses both from 2027-02-18 (PEP 639). The template builds with
   setuptools>=77.