Test aksl_vscanf, and drop a check that could not fail

Coverage caught both. aksl_vscanf was declared, documented and never
called by anything, which is what 100% function coverage is for -- it is
the only metric here that notices a whole function nobody exercises.

aksl_vasprintf had a second vsnprintf whose return it compared against
the first, and a CLEANUP block to free the buffer when they disagreed.
They cannot disagree: the buffer was sized by vsnprintf from the same
format string and the same arguments a few lines earlier. So that was a
failure branch nothing can reach and a free() beneath it that nothing
can execute -- exactly the dead code TODO.md 2.2.11 recorded against the
old aksl_memset and aksl_memcpy, and removing those was the point. The
invariant is a comment now, where it can be read.

Back to 99.5% of lines (1708/1716) and 100% of functions (154/154), with
the eight uncovered lines the ones TODO.md already accounts for.

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

.gitea/workflows/ci.yaml

@@ -92,8 +92,8 @@ jobs:
       # there is no lcov/gcovr to install here.
       #
       # The gate is a ratchet, not a target. Across all four sources the suite
-      # covers 99.5% of lines (1643/1651), 46.0% of branches and 100% of
-      # functions (147/147), so 90/40 fails on a real regression -- a test
+      # covers 99.5% of lines (1708/1716), 46.0% of branches and 100% of
+      # functions (154/154), so 90/40 fails on a real regression -- a test
       # deleted, or new untested code added -- without tripping over rounding.
       # The report is printed either way; the uncovered lines it lists are the
       # missing tests.