Test the libc wrappers: 52% -> 99% line coverage

Every wrapper outside the list and tree code was untested. Six new test
files close that, following the plan already written in TODO.md 1.2-1.6:

  test_stream.c   fopen/fread/fwrite/fclose -- happy paths, the round
                  trip, AKERR_EOF on a short read, AKERR_IO on a stream
                  opened in the wrong mode, ENOENT, and the NULL guards
  test_format.c   printf/fprintf/sprintf -- text *and* count asserted
                  (stdout is pointed at a temp file to check aksl_printf),
                  all eight NULL guards, EBADF on a read-only stream, and
                  512 variadic calls in a loop as sanitizer cover for the
                  missing va_end
  test_convert.c  ato{i,l,ll,f} happy paths, negatives, leading
                  whitespace, NULL guards
  test_path.c     realpath on a file and on a symlink, both compared
                  against realpath(3) since TMPDIR may itself be a link;
                  ENOENT, ENOTDIR, NULL path
  test_strhash.c  djb2 known-answer vectors, len == 0, embedded NUL,
                  stability, NULL guards
  test_convert_strict.c
                  known-failing (2.1.5): the AKERR_VALUE / ERANGE
                  contract the ato* family cannot express today

test_tree.c gains the BFS AKERR_NOT_IMPLEMENTED contract, NULL arguments,
and a callback error that is not AKERR_ITERATOR_BREAK propagating out.

Tests deliberately say nothing about behaviour TODO.md records as
defective -- unchecked ptr/mode/resolved_path, short transfers reported as
success, *count left at -1, the djb2 sign extension -- so the eventual fix
does not have to come with a test rewrite. Each failure case in
test_path.c passes a zeroed buffer, because the wrapper's own error path
formats resolved_path with %s (2.1.6).

aksl_capture.h gains aksl_temp_file() with an atexit unlink backstop.
Without it every test that fails before its own unlink leaves temp files
behind -- which is the normal case for a known-failing test, and happens
173 times over in a mutation run.

Coverage on src/stdlib.c: 52.0% -> 99.0% of lines (200/202), 23.6% ->
51.0% of branches, 8/21 -> 21/21 functions. The two uncovered lines are
both `} HANDLE(e, AKERR_ITERATOR_BREAK) {`, where the macro starts with
the `break;` of PROCESS's `case 0:` arm -- reachable only via a non-NULL
error context whose status is zero, the pathology 2.2.1 exists to remove.

Mutation score on src/stdlib.c: 46.8% -> 89.6% (155/173 killed). CI, the
pre-push hook and the docs ratchet from 40 to 80 accordingly, and the 18
survivors are grouped by cause in TODO.md and README.md. A new CI
coverage job gates at 90% lines / 45% branches.

Verified:
  ctest --test-dir build            # 12/12
  ctest --test-dir build-asan       # 12/12 under ASan + UBSan
  ctest --test-dir build-coverage   # 14/14, report attached
  ctest --test-dir build -j8 --repeat until-fail:3
  gcc -Wall -Wextra -c on all nine test files  # no warnings
  python3 scripts/mutation_test.py --target src/stdlib.c  # 89.6%
No temp files left in /tmp after any of the above.

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

tests/test_tree.c

@@ -99,6 +99,70 @@ static int test_dfs_postorder(void)
     return search_finds_hidden_value(AKSL_TREE_SEARCH_DFS_POSTORDER);
 }
 
+/*
+ * Both breadth-first modes are declared in the header but not implemented, and
+ * say so through AKERR_NOT_IMPLEMENTED rather than by silently visiting nothing.
+ * TODO.md 2.2.10 tracks implementing them; until then this is the contract.
+ */
+static int bfs_reports_not_implemented(uint8_t searchmode)
+{
+    aksl_TreeNode tree[MAX_LEAVES];
+    TreeSearchParams parms;
+
+    build_tree(tree, &parms);
+
+    AKSL_CHECK_STATUS_MSG_CONTAINS(
+	aksl_tree_iterate(&tree[0], &find_value, NULL, NULL, searchmode,
+			  &parms, NULL),
+	AKERR_NOT_IMPLEMENTED, "Searchmode");
+    AKSL_CHECK(parms.steps == 0);
+    AKSL_CHECK(parms.node == NULL);
+    return 0;
+}
+
+static int test_bfs_is_not_implemented(void)
+{
+    return bfs_reports_not_implemented(AKSL_TREE_SEARCH_BFS);
+}
+
+static int test_bfs_right_is_not_implemented(void)
+{
+    return bfs_reports_not_implemented(AKSL_TREE_SEARCH_BFS_RIGHT);
+}
+
+static int test_iterate_null_arguments(void)
+{
+    aksl_TreeNode tree[MAX_LEAVES];
+    TreeSearchParams parms;
+
+    build_tree(tree, &parms);
+
+    AKSL_CHECK_STATUS_MSG_CONTAINS(
+	aksl_tree_iterate(NULL, &find_value, NULL, NULL,
+			  AKSL_TREE_SEARCH_DFS_PREORDER, &parms, NULL),
+	AKERR_NULLPOINTER, "root");
+    AKSL_CHECK_STATUS_MSG_CONTAINS(
+	aksl_tree_iterate(&tree[0], NULL, NULL, NULL,
+			  AKSL_TREE_SEARCH_DFS_PREORDER, &parms, NULL),
+	AKERR_NULLPOINTER, "iter");
+    return 0;
+}
+
+/* A callback error that is not AKERR_ITERATOR_BREAK reaches the caller. */
+static int test_iterate_propagates_callback_error(void)
+{
+    aksl_TreeNode tree[MAX_LEAVES];
+    TreeSearchParams parms;
+
+    build_tree(tree, &parms);
+    /* find_value raises AKERR_NULLPOINTER when it is handed no data. */
+    AKSL_CHECK_STATUS_MSG_CONTAINS(
+	aksl_tree_iterate(&tree[0], &find_value, NULL, NULL,
+			  AKSL_TREE_SEARCH_DFS_PREORDER, NULL, NULL),
+	AKERR_NULLPOINTER, "data");
+    return 0;
+}
+
 int main(void)
 {
     int failures = 0;
@@ -109,5 +173,10 @@ int main(void)
     AKSL_RUN(failures, test_dfs_inorder);
     AKSL_RUN(failures, test_dfs_postorder);
 
+    AKSL_RUN(failures, test_bfs_is_not_implemented);
+    AKSL_RUN(failures, test_bfs_right_is_not_implemented);
+    AKSL_RUN(failures, test_iterate_null_arguments);
+    AKSL_RUN(failures, test_iterate_propagates_callback_error);
+
     AKSL_REPORT(failures);
 }