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>

CMakeLists.txt

@@ -175,8 +175,13 @@ install(FILES
 #                             "unexpectedly passed" -- that is the cue to move
 #                             it up into AKSL_TESTS.
 set(AKSL_TESTS
+    convert
+    format
     linkedlist
     memory
+    path
+    stream
+    strhash
     tree
 )
 
@@ -184,6 +189,7 @@ set(AKSL_WILL_FAIL_TESTS
 )
 
 set(AKSL_KNOWN_FAILING_TESTS
+    convert_strict        # TODO.md 2.1.5 -- ato* cannot report a bad conversion
     list_append_chain     # TODO.md 2.1.1 -- append truncates lists of 2+ nodes
     list_iterate_head     # TODO.md 2.1.2 -- iterate starts at the midpoint
     tree_iterate_break    # TODO.md 2.1.3 -- ITERATOR_BREAK does not stop a walk