Bump libakgl to 42b60f7: every gap this port filed is now closed

All four capability gaps filed against libakgl have landed upstream, so nothing
in this repository is waiting on that library any more.

  text measurement   -> akgl_text_measure, akgl_text_measure_wrapped
  immediate drawing  -> akgl_draw_point/_line/_rect/_filled_rect/_circle/
                        _flood_fill/_copy_region/_paste_region
  audio              -> akgl_audio_init/_tone/_envelope/_waveform/_volume/
                        _stop/_voice_active/_mix
  console input      -> akgl_controller_poll_key, akgl_controller_flush_keys

The signatures came back close to what was filed, and two details are worth
recording for whoever writes src/sink_akgl.c next. The draw calls take an
akgl_RenderBackend * as their first argument rather than reaching for a global
renderer, which fits goal 3's rule that the interpreter draws through whatever
renderer the host already initialized. And the audio API is a synthesised-voice
one -- a tone on a numbered voice plus a separate ADSR envelope -- which is the
shape PLAY and ENVELOPE actually need, rather than the sample playback
SDL3_mixer would have offered.

Checked rather than assumed: libakgl's status band is still 256-260, so the
coordinated range map in CLAUDE.md is unaffected and akbasic's 512-767 does not
move.

Documentation that described these as open is corrected in the same commit --
section 7's gap list, section 3's "known gap" note, the priority list, the
dependency table, and the README's unimplemented-verbs section all said blocked
and no longer are. Section 3 also gains the thing that replaced the gap: the
character grid comes from akgl_text_measure(font, "A", &w, &h), the direct
equivalent of the font.SizeUTF8("A") at basicruntime.go:96, and
akgl_text_measure_wrapped takes the same wraplength argument
akgl_text_rendertextat does so the measurement and the draw cannot disagree
about where a line breaks.

One caveat recorded rather than papered over: -DAKBASIC_WITH_AKGL=ON has never
been configured in this repository, because until now there was nothing to build
against. The akbasic_akgl target is unproven and needs libakgl's own submodules
present. Expect to fix something there on the first attempt.

Default build unaffected: ctest 61/61, doxygen exits 0, no warnings.

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

README.md

@@ -410,7 +410,7 @@ Plus six more the original already carried: a leading `0` selects base 8, so `PR
 * `USING`, `VERIFY`, `VOL`, `WAIT`, `WIDTH`, `WINDOW`
 * The I/O-channel variants (`GETIO`, `INPUTIO`, `OPENIO`, `PRINTIO`, `RECORDIO`)
 
-Four of those are blocked on capabilities `libakgl` does not have yet — text measurement, immediate-mode drawing, audio, and a non-blocking keystroke read. Rather than work around them here, they are filed in [`deps/libakgl/TODO.md`](deps/libakgl/TODO.md) under "API gaps blocking akbasic", with the entry point each one wants.
+None of those is blocked on a missing `libakgl` capability any more. Four were — text measurement, immediate-mode drawing, audio, and a non-blocking keystroke read — and rather than work around them here they were filed in [`deps/libakgl/TODO.md`](deps/libakgl/TODO.md) under "API gaps blocking akbasic". All four have since landed upstream (`akgl_text_measure`, the `akgl_draw_*` family, `akgl_audio_*`, and `akgl_controller_poll_key`), so what is left is akbasic-side work.
 
 ## Deliberately out of scope