Consume libakgl 0.3.0: every workaround deleted, two capabilities gained

0.3.0 closed all ten API gaps this port had filed. The four workarounds go
with them: the CMake block that declared libakgl's vendored dependencies by
hand, the akgl/actor.h include in three files, and the six vtable pointers
assigned by hand in two more, now akgl_render_bind2d().

Two gaps were capabilities rather than inconveniences, and both are now real:

The line editor takes the composed UTF-8 text the ring carries in preference
to the keycode, so shifted characters, keyboard layouts, compose keys and dead
keys all work. A double quote can be typed, which means a BASIC string literal
can be typed -- the sharp end of the old limitation. Letters are no longer
folded to upper case.

SOUND's dir/min/step reach akgl_audio_sweep instead of being refused. dir 3
sweeps once rather than oscillating and TODO.md section 5 says so. A backend
with no sweep still refuses the swept note and plays the held one.

The adaptors now carry an AKGL_VERSION_AT_LEAST(0, 3, 0) floor, verified by
temporarily demanding 0.4.0 and watching it fire.

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

README.md

@@ -464,7 +464,7 @@ The exception is `FILTER`, which sets the SID's filter cutoff, band switches and
 
 * [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) 0.2.0 — 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.
+* [libakgl](https://source.starfort.tech/andrew/libakgl) 0.3.0 — **optional**, only for `-DAKBASIC_WITH_AKGL=ON`. Pulls in SDL3. 0.3.0 closed every API gap this project had filed against it, so there are no libakgl workarounds left here.
 * [basicinterpret](https://source.starfort.tech/andrew/basicinterpret) — the Go original. Vendored as the behavioural spec for questions about semantics, and for nothing else: its acceptance corpus and its Commodore font are checked in here now, so **the build and the test suite do not need it**. Not linked, not built, and safe to omit.
 
 Everything is a submodule; `git submodule update --init --recursive` gets all of it. There is nothing to install first.