Consume libakgl 0.2.0 and libakstdlib 0.2.0

Rebasing the defect filing onto libakgl's origin/main brought two upstream
commits with it -- "Consume libakstdlib 0.2.0" and "Bump libakgl to 0.2.0" --
and libakgl's sources now use the 0.2.0 akstdlib API. Our CMakeLists declares
akstdlib::akstdlib from our own tree first, so libakgl was compiling 0.2.0-era
code against 0.1.0 headers and failing on aksl_fwrite, aksl_fread and
aksl_realpath. The two have to move together.

The code change is one call site. aksl_fwrite now takes a required size_t
*nmemb_out and reports a short transfer as AKERR_IO rather than a silent
success, so DSAVE onto a full disk is an error a program sees where before it
reported nothing. The count is passed and discarded on purpose: the library does
the noticing now.

The documentation change is larger, because libakstdlib 0.2.0 fixed all six of
the confirmed defects TODO.md section 1.9 was built around. That section was a
table of bans; leaving it would send an agent around a workaround for functions
that now work. The aksl_ato* family raises AKERR_VALUE on no digits or trailing
junk and ERANGE on overflow -- exactly the contract that section demanded --
aksl_list_append no longer truncates, aksl_list_iterate no longer skips the
first half, AKERR_ITERATOR_BREAK stops a tree traversal, and aksl_realpath no
longer reads uninitialised memory. One caveat survives: aksl_strhash_djb2 still
sign-extends char, which section 1.3 already covers and the symbol tables still
cannot reach.

src/convert.c has therefore outlived its reason, and is deliberately left in
place. Its own note said to delete it when libakstdlib grew the contract, and
that condition is now met -- but doing it touches four call sites, changes the
raised status from AKBASIC_ERR_VALUE to AKERR_VALUE where section 1.8 says
message text is part of the acceptance contract, and would silently gut the CI
mutation job, which is bounded to src/convert.c and src/symtab.c. Section 1.9
now lists all of that. Worth doing on purpose rather than as a side effect of a
version bump.

libakgl defect #14 is closed by its own 1066ac7, which bumped it to 0.2.0 while
this was being written. The requirement is no longer pinned by submodule commit
in the README, because AKGL_VERSION_AT_LEAST(0, 2, 0) finally means something.

70/70 core, 71/71 with libakgl, 70/70 under ASan+UBSan, clean under
-Wall -Wextra, doxygen clean.

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

README.md

@@ -433,10 +433,10 @@ The exception is `FILTER`, which sets the SID's filter cutoff, band switches and
 # Dependencies
 
 * [libakerror](https://source.starfort.tech/andrew/libakerror) 1.0.0 — TRY/CATCH-style error contexts. Every function that can fail returns one.
-* [libakstdlib](https://source.starfort.tech/andrew/libakstdlib) 0.1.0 — libc wrappers that report through `libakerror`.
-* [libakgl](https://source.starfort.tech/andrew/libakgl) 0.1.0 at commit `42b60f7` or later — **optional**, only for `-DAKBASIC_WITH_AKGL=ON`. Pulls in SDL3. The requirement is pinned by submodule commit rather than by version on purpose: 42b60f7 added 22 public symbols and left the version and the `libakgl.so.0.1` soname alone, so `AKGL_VERSION_AT_LEAST(0, 1, 0)` cannot tell the two trees apart. Filed upstream; `TODO.md` section 8 has the detail.
+* [libakstdlib](https://source.starfort.tech/andrew/libakstdlib) 0.2.0 — libc wrappers that report through `libakerror`.
+* [libakgl](https://source.starfort.tech/andrew/libakgl) 0.2.0 — **optional**, only for `-DAKBASIC_WITH_AKGL=ON`. Pulls in SDL3, and requires `libakstdlib` 0.2, which is why the two move together.
 * [basicinterpret](https://source.starfort.tech/andrew/basicinterpret) — the Go original, vendored as the behavioural spec and the acceptance corpus. Not linked, not built.
 
 Everything is a submodule; `git submodule update --init --recursive` gets all of it. There is nothing to install first.
 
-Note that `libakstdlib`'s `aksl_atoi`/`atol`/`atoll`/`atof` family is deliberately **not** used here, because it cannot report a conversion failure — `atoi("not a number")` returns success with `0`. `src/convert.c` wraps `strtoll`/`strtod` with the strict contract instead, and `TODO.md` section 1.9 records which of that library's calls are cleared for use. That file will be deleted when `libakstdlib` grows the wrappers its own `TODO.md` section 3.1 already calls for.
+`src/convert.c` wraps `strtoll`/`strtod` with a strict contract — no digits consumed, trailing junk and overflow are all errors — because `libakstdlib`'s `aksl_atoi` family could not report a conversion failure at all. As of `libakstdlib` 0.2.0 it can, so that file has outlived its reason and is due for removal; `TODO.md` section 1.9 lists what deleting it touches, including the CI mutation job that currently targets it.