Version at 0.2.0: complete the wishlist, document it, gate the docs

Closes what was left of TODO.md sections 1, 2 and 3, and rewrites that
file to hold outstanding items only.

The API break gets a minor bump, because pre-1.0 the soname carries
MAJOR.MINOR and 0.1 and 0.2 are therefore different ABIs. Five
signatures changed and the ato* contract with them; UPGRADING.md is new
and lists every one, with the before/after for the cases the compiler
cannot warn about.

Section 3.1 is finished: reallocarray with the multiplication checked,
aligned_alloc and posix_memalign, asprintf/vasprintf, scanf/vscanf.
Four functions on that list are deliberately absent rather than missing
-- sprintf, strtok, setbuf and perror -- and TODO.md now says which and
why, so nobody adds them thinking they were forgotten.

Section 1.9, the cross-cutting tests:

  tests/test_pool.c    drives every failure path AKERR_MAX_ARRAY_ERROR
                       + 10 times and checks the pool after each round,
                       because a wrapper that leaks a slot fails a
                       hundred calls later in unrelated code. It also
                       asserts that each error names the function and
                       file it was raised from, which is what catches a
                       FAIL that migrates into a helper during a
                       refactor: status right, message right, origin
                       quietly lying.
  tests/negative/      two sources that must FAIL to compile, built with
                       -Werror and registered WILL_FAIL. AKERR_NOIGNORE
                       and the format attributes are enforced by the
                       compiler and by nothing else; drop either and
                       every ordinary test still passes.

Thread safety is answered rather than tested: the library is not
thread-safe and cannot be made so from here, because libakerror's error
pool is an unlocked process-global array. README.md says so plainly and
TODO.md carries it as the item blocking any future pthread wrappers.

Doxygen is configured and gated. All 147 public functions have @brief,
a @param each, @throws per status and @return; EXTRACT_ALL is off and
WARN_NO_PARAMDOC on, so `cmake --build build --target docs` fails on an
undocumented entity. It ran to 0 warnings. The Doxyfile carries no
version -- cmake/RunDoxygen.cmake feeds PROJECT_NUMBER in from
project(), so that stays the one place a version is written.

CI now builds against the submodule it pins instead of also installing
libakerror@main and never linking it, adds -Werror, and gains a
sanitizer job. The pre-push hook matches, and runs the docs check too.

Coverage: 99.5% of lines (1643/1651), 100% of functions (147/147). The
eight uncovered lines are each uncovered on purpose and TODO.md says
which and why.

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

tests/negative/noignore.c

@@ -0,0 +1,26 @@
+/*
+ * NEGATIVE COMPILE TEST -- TODO.md section 1.9.
+ *
+ * This file must NOT compile. It is built by the CTest entry `negative_noignore`
+ * with -Werror, and that test is marked WILL_FAIL, so a successful build is a
+ * test failure.
+ *
+ * What it asserts: AKERR_NOIGNORE is actually attached and actually effective.
+ * Every entry point in this library returns an error context the caller must
+ * look at, and the whole design rests on discarding one being hard rather than
+ * merely inadvisable. AKERR_NOIGNORE expands to warn_unused_result, which does
+ * nothing at all if it is dropped from a declaration during a refactor -- and
+ * nothing about the resulting library would look wrong.
+ */
+
+#include <akstdlib.h>
+
+int main(void)
+{
+    void *ptr = NULL;
+
+    /* Discarding the returned context. This is the line that must not build. */
+    aksl_malloc(32, &ptr);
+
+    return 0;
+}