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>

TODO.md

@@ -11,11 +11,11 @@ second, what is missing last.
 
 | | |
 |---|---|
-| Wrapped | 147 public functions across `src/stdlib.c`, `src/string.c`, `src/stream.c`, `src/collections.c` |
+| Wrapped | 154 public functions across `src/stdlib.c`, `src/string.c`, `src/stream.c`, `src/collections.c` |
 | Tests | 17 CTest binaries plus 2 negative-compile entries, green under the default and sanitizer builds |
-| Line coverage | 99.5% (1643/1651) |
-| Function coverage | 100% (147/147) |
-| Doxygen | 100%, gated — `cmake --build build --target docs` fails on an undocumented function, parameter or return |
+| Line coverage | 99.5% (1708/1716) |
+| Function coverage | 100% (154/154) |
+| Doxygen | 100% of 154, gated — `cmake --build build --target docs` fails on an undocumented function, parameter or return |
 
 The six confirmed defects that used to head this file are fixed and
 `AKSL_KNOWN_FAILING_TESTS` is empty. What they were, and what changed as a