ci: a failed KTLO leg turns main and pull requests red; report it as a warning and exit 0 #1672

Problem. Compat Python 3.7 (KTLO) fails on every run since #1665, because 3.7 cannot compile pcapkit. Compat's KTLO legs use job-level continue-on-error (python-compatibility.yml:41). The workflow run still succeeds, but the job's check run concludes failure, so main and every PR show a red X. The test and integration KTLO legs use step-level continue-on-error, so their jobs conclude success; they still emit ::error annotations (unit-tests.yml:163, :413 and :933).

Ruling (the owner, in chat): a KTLO failure must not turn main or a PR red. The leg exits 0 even when its tests fail, and the failure stays visible.

Fix. Every KTLO leg concludes success. It reports a failed step as a ::warning annotation plus a line in the job summary. Supported legs still fail exactly as they do now, and a test pins both behaviours. Release status's KTLO notice stays.