[#39] CI: test the stale-entry notice and the CR escape of the baseline check

[vharseko]

Fixes #39 — the third review round of #28 (review 5389582272), done on master, where the code arrived through #38.

Changes

.github/workflows/build.yml:

• Self-test the baseline check: a known entry that is gone must give exactly one ::notice title=No longer found; … line and exit 0.
• Self-test the baseline check: a message 50% + CR, as a new Antora error and as a stale known one, must appear as 50%25%0D in both the ::error and the ::notice line.
• Both self-tests: on failure, the output of check.sh / head-meta.sh goes to stderr quoted with > , because the runner reads workflow commands on stderr too and its ::error lines would become annotations of the pull request. This is done in all three places, including the two that [#36] [#37] CI: check link anchors, head meta tags and the robots.txt Sitemap line #38 added.

Verification

The steps were cut out of build.yml and run locally in an Ubuntu container (bash 5, jq, node):

• both self-tests pass on the head;
• the baseline self-test fails against each of these mutants: no failed=true, swapped comm operands, .errormap in broken-links.jq, comm -13 → comm -12, fixed=, ::notice → ::debug, and the CR escape deleted in both places, in the error only, or in the notice only; the head meta self-test fails against a broken head-meta.sh;
• when either self-test fails, no line of its stderr starts with ::.

The <br> replacement in the "Errors and warnings" table of the job summary stays untested, as noted in the review.