Record that there is no directory-reading wrapper

opendir/readdir/closedir have no aksl_* counterpart, so a consumer that
wants to list a directory has to call them itself and report through errno
in a file where everything else reports through an akerr_ErrorContext *.

Found from akbasic, which needs it for Commodore BASIC's DIRECTORY verb and
refuses the verb rather than working around the gap.

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

TODO.md

@@ -355,3 +355,23 @@ lists and no trees, drawing everything from fixed pools by design. A consumer th
 does allocate would weight §4.1's `open`/`read`/`write` far higher than this one
 does. The next thing worth doing is porting akbasic onto this release and
 counting the calls again.
+
+## Requested by consumers
+
+1. **No directory-reading wrapper.** There is no `aksl_opendir` / `aksl_readdir` /
+   `aksl_closedir`, so a consumer that wants to list a directory has to call
+   `opendir(3)` itself and step outside the error convention every other call in
+   its file follows — reporting through `errno` where everything around it
+   reports through an `akerr_ErrorContext *`.
+
+   `akbasic` hit this implementing Commodore BASIC's `DIRECTORY` verb, and
+   refuses the verb rather than working around it: `src/runtime_disk.c` reports
+   "DIRECTORY is not implemented: libakstdlib has no directory-reading wrapper
+   yet". The shape it wants is the one the `aksl_f*` family already has — a
+   handle out through a pointer parameter, `NULL` on success, `ENOENT` and
+   `EACCES` propagated as themselves.
+
+   A wrapper would want tests for: a directory that does not exist, one that
+   cannot be searched, an empty one, and one whose entries outlast a single
+   read — plus whatever the `d_type` portability story turns out to be, since
+   not every filesystem fills it in.