The Python 3.10 tests stay on Ubuntu 24.04

Python 3.10 reaches end of life in October 2026, and the newer image that
ubuntu-latest moves to may not offer it. The leg would then fail on setup,
and the badge on the public README would turn red with nothing broken.
Pinning it keeps 3.10 tested for as long as MeshTerm says it supports it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -59,8 +59,12 @@ jobs:
         # shows up. It was shaped this way when the repository was private and
         # minutes were metered; on a public one they are not, so this can widen to
         # every OS times every version whenever the wait for a full run is worth it.
+        #
+        # 3.10 stays on Ubuntu 24.04. It reaches end of life in October 2026, and a
+        # newer image that ubuntu-latest moves to may not offer it at all. That would
+        # turn the badge red with nothing broken. Drop this leg when the floor rises.
         include:
-          - { os: ubuntu-latest, python: "3.10" }
+          - { os: ubuntu-24.04, python: "3.10" }
           - { os: ubuntu-latest, python: "3.11" }
           - { os: ubuntu-latest, python: "3.12" }
           - { os: ubuntu-latest, python: "3.13" }